Compared to....say, SecuROM, how bad is Tages?
Will it fuck up my computer horribly, cause I REALLY want to play XIII on my computer knowing that it wont get destroyed in the process.

Never had a problem with Starforce personally. I don't mind the disk check version, even if it does require an update to work with Vista.
I bought a download version of Call of Juarez a few months ago (gamesplanet), that uses an activation server version of Starforce. Not a fan of activations in general when it comes to games. Gamesplanet are very poor btw...I needed a patch for CoJ and asked them if they were going to release one as their site, because they say they have to specially package them. They said just keep checking their site, and in the meantime I discovered I could use the retail patch, no special packaging required whatsoever.

I do have other things to do sometimes you know. ;)
Tages actually isn't one I'm particularly familiar with--I've only used one or two games with it and they had only the disc-based version. Disc-based Tages seems to be very weak since for at least some games the pirates didn't even need to make a crack; mounting the image is seemingly sufficient without even needing to cloak the virtual drive (as with disc-based SecuROM/StarForce).

Oh right, you're an astrophysicist for NASA while working nights as a sysadmin (which is usually harder than being a rocket scientist if you do it right).

Playing XIII won't harm your machine. Ubisoft officially patched out the protection in XIII with the v1.3 patch - it's like an official crack. If you're installing it from the original physical media, apply v1.01 patch followed by the v1.3 patch (Official Ubisoft CD Protection removal). After the official v1.3 patch is applied, the protection will be removed and you can just run the xiii.exe without a disc.
For those on Vista/Win7 64 bit, the installer is incompatible. To get around this, install the game in an Win XP virtual machine and apply the 2 patches mentioned above. After this the protection will be removed and you can just copy the game folder anywhere back to the 64 bit host machine and run xiii.exe to play - the game won't need registry settings. To avoid install hassles in the future you can just archive that game folder.
For those that haven't played it it's well worth it. GamersGate has it for $5 USD.
You can also run XIII in widescreen. The pre-rendered cutscenes will be stretched but in game it's proper aspect. Here's the intro and first level for reference.

I meant will Tages fuck up my computer since I tested XIII and it runs fine.

Short answer: probably not.
Tages doesn't mess up optical drives (something StarForce was notorious for) nor does it seem to care about "piracy" software (Nero, Process Monitor, etc.) to the degree that some SecuROM versions do. Most problems seem to be related to activation, but that can happen with any DRM system (even Steam), and if you have a game that only uses it for a disc check that's not an issue anyway.

The tages driver shouldn't harm your computer. If for some reason you thought tages was causing your machine problems, you can easily remove the tages driver. As mentioned earlier, tages was officially patched out of retail XIII with the v1.3 patch so it isn't even needed to run the latest version of it.

Is this the case for all physical versions (e.g. rerelease)?
I know this is the case with the original release, I bought an old copy this month for £1.99 and patched it so it doesn't need the disk :)