First, is there a good site for searching for motherboards, that will allow them to be filtered in useful ways (like USB support and video outputs, for example)?

Second, what motherboards would you suggest I look into? Requirements:
* CPU is an AMD Ryzen 3400g. The motherboard needs to support this with the factory BIOS (I don't want to have to locate another CPU just to update the BIOS).
* DisplayPort (so that I can save my monitor's HDMI input for other devices).
* A USB-C port with Power Delivery, preferably able to power a Raspberry Pi 4(00); this must not be the same port that's used for DisplayPort, and I would prefer this to be on the front.
* Will want more USB, including USB 3.0. Also, SATA. PS/2 would be nice so I can re-use my current (typewriter-type, not piano-type) keyboard without having to hunt for the adapter it came with.
* AMD-V for virtualization is mandatory. AMD-VI support for PCI pass-through would be nice. (Note that the device I decide to pass through might be something other than a video card.)
* Motherboard shouldn't create unnecessary visual or audio noise. (For instance, I don't want any RGB here that's on by default.)
* Must be well supported under Linux. (Windows will probably not touch this machine, and if I end up needing it down the road, it might very well be limited to a VM.)
(Note that I'm not actually concerned about features like overclocking; while it might be nice, it's not something I necessarily intend to do, and I would, of course, need good cooling for that to work.)

www.newegg.com

When you first go to the Motherboard section you'll only see a limited number of filters on the left. But once you pick a platform (AMD / Intel) then much more detailed filter options start showing up.

As for a recommendation - you can use the filters just as easily as I can - but I will make a few general comments on your requirements that you may want to take into consideration.

For your use case, I'd recommend looking at a B550 chipset. This should support the 3000 series chipset out of the box and provide support for newer PCIe and USB technologies. Technically, you could save some money and use a B450 board with your Ryzen 3000 series CPU... but there a several caveats with that path (i.e. "it SHOULD work, but..."). If you want to be sure everything works out of the box with no fuss, then spend the extra 50 bucks on a B550 board to avoid any potential headaches or surprises. The X570 platform is also an option, but I feel like that would be overkill for what you're trying to do (it's intended more for the extreme/enthusiast crowd, whereas you seem more like the tech-geek demographic -- which the B550 squarely fits into).

Native DisplayPort support on the motherboard isn't quite as common as HDMI or DVI, since DisplayPort is a proprietary standard. But you can purchase an inexpensive DVI- or HDMI-to-DisplayPort adapter to easily work around that. Don't let native DisplayPort support be a limiting factor -- you can work around that "problem" with a $15 adapter.

Any modern motherboard with USB-C support should provide power delivery. However, you'll want to ensure your case's USB-C connectors also supports power delivery. Some low-end cases cheap out on their front-panel circuitry and only include USB-C output.

USB 3.0 and SATA support should be plentiful on any modern motherboard. PS/2 support, however, is becoming quite the rarity. If you really want to insist on a motherboard that has native PS/2 support, I fear you'll be missing out on much better boards that excel in every other area. As with the DisplayPort, I think you'd be much better served by dropping this "requirement" and simply purchasing a $5 PS/2-to-USB adapter.

AMD-V / Vi is determined by your CPU, not your motherboard. Your 3400g supports both AMD-V and AMD-Vi, so you're golden.

Motherboard shouldn't create unnecessary visual or audio noise. (For instance, I don't want any RGB here that's on by default.) -- If you're talking about RGB lighting and such, you can read the product description and see whether it has RGB effects (or just look at the product picture and it should be immediately obvious).

Linux support really shouldn't be an issue since Linux already has solid support for the B550 platform and you're explicitly NOT looking for some "exotic" motherboard with a bunch of non-standard features. As long as you get a bog-standard B550 board, then you should be golden on any current distro. If you REALLY want to be sure, go to the board manufacturer's website and see if they have native Linux drivers for your board. But even if they don't, as long as you're on a current hardware platform it's almost assured that any serious Linux distro will be supported.

Side Note: while you're not currently interested in overclocking, your CPU can be overclocked a fair bit if you upgrade from the stock cooler. Perhaps an interesting side-project to undertake sometime in the indeterminate future. :-)

(OK, that was a lot more than "a few general comments". Hope that helped -- good luck with your build!)

(EDITED to correct mistakenly referring to the Ryzen 3000 series as "third-gen Ryzen")

The 3400G is a second gen though, and that's not officially supported by B550. (Also, B550 is not guaranteed to have 3rd gen support OOTB, you might need to use an older CPU or BIOS flashback if your board supports it)

Official chipset compatibility chart: https://www.amd.com/en/chipsets/b550

USB-C PD might be hard to find in a typical desktop board, however, pretty much any USB-C port should be able to power the Pi without issue.
Good luck finding any info about USB spec without e-mailing the vendor directly. There's plenty of info on RGB though.
Look for marketing slangs like "Asus Super Ultra USB POWA" or "Gigabyte Mega USB charging" Or "MSI,s 10x fast chrage)

Yep, more specifically Zen+.

https://www.newegg.com/amd-ryzen-5-3400g/p/N82E16819113570
https://www.windowscentral.com/amd-ryzen-5-3400g-review?amp
https://en.wikichip.org/wiki/amd/ryzen_5/3400g

B450, X470 and X570 should work great with the 3400G APU. Personally, I think ASrock and Asus are good since they seem to be less likely to limit their boards, and I do think you can easily turn RGB off on most boards. Again, as I wrote in the other thread, if USB-C is a must then look at this list: https://thunderboltlaptop.com/usb-c-motherboards/

This is the only mamaboard I could find that has DP+PS/2+USB-C, and it's a combo with 3400G so I guess it's supported OOTB:
https://www.amazon.com/Gaming-X570-Plus-Motherboard-AMD-8-Thread/dp/B07Y35QB18

It was mentioned here also:
https://motherboardsworld.com/best-motherboard-for-ryzen-5-3400g-review-2020/

Speaking of pies:
https://raspi.tv/2019/how-much-power-does-the-pi4b-use-power-measurements

Thanks for the correction, I'm not sure why I was thinking the 3000 series was third-gen Ryzen. Sorry for the misinformation!

I believe I just read something that says that the non-g 3000s are third-gen, but the 3000gs are not, which is a rather confusing way to name things.

It is.
Simplest way to know what is what is to look at the contruction node (lythography) because things can get messy with naming schemes.

14nm is 1st gen Zen
12nm is second gen Zen+
7nm is third gen Zen2 or newed release Zen3 (5*** series)

Wikipedia has a very good article about it

Edit: a bit of explanation: APU's (CPU with graphics) are usually a generation behind compared to non-graphics CPU's because they are based on laptop manufacturing, not desktop. They have less "features" to make space for the graphics on the die.

I recommend getting a mother board with a good sound card, you can tell if it has a bunch of sound ports and a optical out. Thats must me though :) I need it for playing good music. Other than that there really is no difference between them, I have had them all from low end to high end. Especially when you are talking a ryzen, I managed to overclock a 1700x to 1800x levels on a garabge dell motherboard, though I had to put a huge fan on it.

Since you are going with a 3400g there isnt really much reason to overclock anyways, so its not gonna matter just buy the cheapest one you can find if sound isnt important to you.

Heres a cheap one that would work find. It even has optical out :)

https://www.amazon.com/ASRock-B450M-Promontory-CrossFireX-Motherboard/dp/B07MV9BMNY/ref=sr_1_9?crid=1SQVVWKFQPO8J&dchild=1&keywords=b450+motherboard+wifi&qid=1606522659&sprefix=b450+mother%2Caps%2C163&sr=8-9

The only other thing is if you plan on upgrading to a newer ryzen, the 450 boards wont do that.

And, we have a winner! I just love ASrock boards; cheap and still high on performance with lots of features (I have its "brother" B450M-HDV R4.0). Added bonus it got Realtek for both soundchip (ALC892) and netchip (RTL8111H) :)

ASROCK B450M STEEL LEGEND RGB w/RYZEN 5 3400G - Showcase and install
https://www.youtube.com/watch?v=Mu0it5CS92k
https://www.youtube.com/watch?v=hOXHp5nJ9jg

https://www.pctechreviews.com.au/2020/01/30/reviewed-asrock-b450m-steel-legend/
https://thinkcomputers.org/asrock-b450-steel-legend-motherboard-review/
https://pokde.net/review/asrock-b450m-steel-legend-review

Yes they will, at least to Ryzen 5000 series (Zen3) for ASrock boards.

https://www.tweaktown.com/news/76340/amds-new-ryzen-5000-series-cpus-get-surprise-early-b450-mobo-support/index.html
https://www.pcgamer.com/amd-ryzen-5000-b450-bios-update-motherboard/

Data point: This computer has an AsRock H87 Pro4. I would not expect it to be suitable for my new machine (it's pretty old and for the wrong CPU manufacturer), but I think it's served me well all these years. In any case, it is out of stock and the Newegg reviews are from 2013/2014; that should get you an idea of the age of my current computer. (It still does work, though there's some USB interference on the front, and I don't know if there's a connection to a fan that's broken (remember, this machine has cooling issues).)

Would said overclock be stable with the stock cooler, and in particular, would it be stable long enough (without throttling) to compile a kernel (let's assume "make defconfig" followed by "make -j8" on a Linux 5.10 source tree)?

By the way, what does "high speed" mean in this context?

That's really good.

I've had boards that have used poor quality on the components and at least two boards died within 4 years because 1 or 2 capacitors gave in, but yes, also in my experience ASrock lasts longer.

Also, do note, that if you go for that Asrock board make sure its a newer version (just send the shop an email or something). Looking at the CPU support page it looks like there are two kinds of 3400G and earliest BIOS support is 2.10 for 3400G, but that one came out about 1,5 years ago. Even 2.6 is over a year old, so the chance of you getting an old board is slim, but it's something to have in mind IF the computer won't POST...

AM4 Ryzen 5 3400G (YD340GC5M4MFH) 65W Picasso 3.7GHz 2MB B1 P2.60
AM4 Ryzen 5 3400G (YD3400C5M4MFH) 65W Picasso 3.7GHz 2MB B1 P2.10

https://asrock.com/MB/AMD/B450M%20Steel%20Legend/index.asp#CPU
https://asrock.com/MB/AMD/B450M%20Steel%20Legend/index.asp#BIOS

Why not rather buy a pc with an AMD 4000 series processor in it? Maybe they are neigh impossible to find due to AMD's supply issues but should be quite a bit faster and using less power too without the need for overclocking.

Look for a pc with one of these instead of that old 3400G:

AMD Ryzen 5 4600G
AMD Ryzen 5 4600GE
AMD Ryzen 3 4300G
AMD Ryzen 3 4300GE

E models are lower clocked and have a thermal limit of 35W instead of 65W so maybe better avoid.