I've some questions about Windows since I decided to just cave in and pay for an OS license to go with a new rig I'm due to put together soon.
1) Have anybody got experience with XP64? I've heard vague rumours that there are issues with the 64-bit version.
2) What should I get right now, given that I want to stay away from Vista as long as possible? Do I get an upgrade to Windows7 if I buy Vista, making it slightly more bearable?
3) Could anybody properly explain the "OEM"-stuff? If I buy a copy of Windows XP Professional OEM CD, what exactly am I getting?
4) I'm an advanced user and figure I at least need the "Professional" versions of any OS. I use all sorts of networking, remote desktops, and need full system access in general. I've come to believe that the cheaper versions, like XP Home, don't support some of this activity?
A bit newbiesh questions to come from an "advanced user", but I've been a student for 6 years and have never considered buying an OS. All I had to do was download the latest XP Pro install from the University network.

Most of the issues with XP x64 were the same as any other x64 operating system, driver support & software compatibility. SOME applications, notably rather old ones, won't like 64bit. I've been running vista ultimate x64 for... about 18 months and I can count on one hand the number of genuine 64 bit incompatibility issues I've seen. The majority of problems I've had are with games that do hardware checks and then panic because I don't seem to have a Pentium processor capable of 66MHz or better. Foolish old me buying a 2.8GHz core2 duo...
To be honest, you'd be mad to buy XP now, especially if you want to game on this new computer. Vista is not as bad as popular opinion claims, its much like securom. There's legitimate issues but there's also a deep undercurrent of "Something unexpected happened, OMFG! VISTA IS TEH SUXXORZ!!". The main problem a lot of new users have is that several bits of vista are different to xp but most of those things seem to be the same in Win 7 so you might as well get used to it with vista. If you can get a vista licence with the win7 upgrade offer (don't assume you'll get an upgrade, confirm it to be sure), you get 2 OS' for the price of one and once upgraded, you can always use vista to run a media centre PC or something.
OEM licences are designed for system builders, technically supposed to be for the small PC business who build & sell PCs, they make the OS cheaper as an incentive to supply it and increase market share. The software is identical but you might be limited in the number of activations, I've never faced an issue like that though
http://www.winsupersite.com/showcase/windowsxp_home_pro.asp thats a breakdown of the home vs pro difference for xp but its a bit inaccurate, my netbook came with xp home and I've got remote desktop (that might be a service pack thing though). For Win7, you'd almost certainly want Pro because that comes with the XP mode http://windows.microsoft.com/en-us/windows7/products/compare-editions

64-bit as a whole is quite pointless at the moment unless you really have a need for more than 4GB of RAM. I have heard talk of issues with XP64, not quite sure what they are exactly. I do know that XP64 is basically just a botch job though hacked together in a few minutes.
Try Vista if you haven't. Honestly there is nothing wrong with it. Some places like Amazon are offering free copies of Windows 7 if you buy Vista now. I saw a new Vista Ultimate for £80, meaning you're potentially getting Windows 7 Ultimate for £100 less!
OEM-stuff... Basically no difference. You should only get this if you plan on building more computers and selling them, I think.
Yep you're right - XP Home has nothing you'll want. Stick with Professional.
That probably doesn't help... But y'know. At least it's not fanboy driven.

With 1.75GB of dedicated video memory on the card I'm buying, yeah, I will need a 64 bit OS.

XP 64-bit never did get full support from manufacturers. A lot of drivers etc. are only 64-bit with Vista or newer. For this and many other reasons you're better off going with Windows 7.
Windows 7 will reach General Availability on October 22. I suggest you install the release candidate now and then wait for the release version. While you could use Vista instead while you wait for 7, you end up having to reinstall when 7 hits anyway (while Windows 7 supports "upgrade installs" from Vista--but not XP or the 7 RC--it's best to do a clean install). It's also cheaper to use the free RC now and then buy a full version of 7 rather than paying for both a full version of Vista and an upgrade version of 7.
The differences with OEM (or "System Builder") licenses are as follows:
1. No phone tech support (except for activation and other crucial functions)
2. No license migration rights--you aren't allowed to take that license and use it on a significantly different hardware configuration; Microsoft technically considers the motherboard to be what the license is tied to, but in practise the activation system is reasonably flexible and won't care about gradual upgrades as long as some components remain from the last configuration (otherwise it sees it as a completely new system).
3. You don't get the fancy box. ;)
4. You only get media for one architecture; with a retail version you get both architectures in the same box (with the exception of XP 64-bit). Additionally, the license you get only works with the architecture you buy it for.
The OS itself is functionally identical no matter how you buy it. If the above limitations don't bother you feel free to go OEM. However if you like the idea of only paying for a single copy of Windows no matter how many times you build, buy, or upgrade you should go with retail rather than OEM because OEM isn't really meant for such advanced migration and your key may be blacklisted if you try to recycle an OEM license in this manner.
With XP and Vista, some useful features are only found in higher versions (note that the 64-bit version of XP is Professional-only). With Windows 7, however, Home Premium contains all the functionality most users will need (see the product editions comparison for what each version contains). Basically, if you aren't wanting to do advanced things like host Remote Desktop sessions or join a domain you won't need Professional; other features only available in Pro and higher, such as BitLocker, can be replicated by third-party solutions (some of which are free).
With Vista and newer you can use Windows Anytime Upgrade to buy access to a higher product version later on; this is cheaper than buying an Upgrade license and allows you to do an in-place upgrade (it simply unlocks features lying dormant; you don't have to restore your files and programs and everything keeps working as if nothing had happened).

If your building a system from scratch 4gb ram is probably the minimum you want to be thinking of putting into it. I would personally get windows 7 64bit, with the way gaming is headed 64bit is very likely to be the dominate version of windows 7 OS.
I also don't think video card Ram effects your limit on a 32bit system.

Note that the underlying functionality of XP Mode--Windows Virtual PC--is freely available for any version of Windows 7. All you get with Pro and higher is a free XP Pro SP3 VM license. If you have your own usable XP license that isn't an issue.

I only buy OEM and have migrated the same license over three completely different builds. As long as you're not activating it every couple months, you're golden. I can't recall offhand how many activations you get before you have to phone MS, but if it comes to that, all you need to do is tell them your hard drive or motherboard died and had to be changed and they'll activate it without question.
The ONLY reason (IMO) to go retail is if you want tech support by phone from MS, and if you're an advanced user, you probably don't need that. Save your money and buy OEM.

FWIW I've read numerous claims that it does. If this is indeed the case, then with 1.75gb of VRAM you're going to be looking at roughly 1.5gb's of usable system memory. 64bit is definitely the way to go.
Oh and Vista is really rather good - don't believe the (anti)hype. However, it probably is worth going for the free RC of Win 7 and then moving on to the full version.
edit: Regarding the OEM thing - given that you're European, you may find that the pre-order is still the best way to go. I managed to snag a copy of Win 7 pro for £90 - roughly half the price it will be at retail. Had I gone for home premium it would have been £45 - about a third of the price. It seems the US pre order is closed, the UK is still open, dunno about the Norwegian on. I flicked the MS site over to Norwegian locale but got a bit lost after that :).

It does. A 32 bit system can only address 4 GB of memory in total. Your video RAM (all or part of it) will be included in that 4GB of address space.

OEM licenses are not transferable! From the EULA:
"The software license is permanently assigned to the
device with which you acquired the software. That device is the “licensed device.” ... You may install one copy of the software on the licensed device ... You may
not use the software on any other device."
Microsoft is very generous in terms of what upgrades you can make to the originally-licensed system, but using that license on a completely new system is a breach of the license agreement and abuses Microsoft's generosity. If you want to take a single copy of Windows between different systems, buy a retail license. Retail licensing allows you to use it on as many systems as you like as long as it is only in actively use on one system at a time.
If you want to abuse Microsoft's generosity and outright lie to their telephone staff that is your business, but don't go acting as if this is an acceptable course of action for others to take.

As far as I'm concerned, an OEM licence should be per user rather than per device. I never give people copies of my software or install more than once but as long as I'm using the one licence at a time on hardware that I own, I think that should be acceptable. Not that my opinion matters to the contents of the EULA, naturally
Actually it can't address a full 4GB of ram, it stops off at a bit over 3.6 unless I'm remembering wrong

As for the memory limit it's 4 gig of addressable space. Everything that makes a call to memory has to be taken out of that 4 gig. From VRam to your BIOS. When you add everything up it is 4 gig but you'll NEVER see the full 4 GB on a 32 bit system because of everything else that requires memory addressing space.

Whatever floats your boat. As I said, I've migrated my OEM XP to three different computers (it's installed on only one system at a time, by the way), and never had any issues from Microsoft about it. And from my understanding, they're fully aware that people do this.