Hi xxxxsh4d0wxxxx.
My 2 cents:
Rridgely is right (of course), XP is faster, and by now, more secure, but AMD dual core + 2 gig ram will run most anything.
I have never installed a new OS over an existing one, but I would several questions if I had to make the decision you are considering.
1. If I install XP on my new computer, then thats it, right? Microsoft contends that it is stuck on that computer forever. I don't intend to try to hack it so that I can use it on more than one, don't even know how.
2. If I install my XP on my new computer, does that prevent me from reinstalling Vista on my new computer if I want to later?
3. If I do reinstall vista, will Microsoft registration still think that my copy of XP is associated with my new computer (now running vista again), so that it is now uninstallable?
4. Is vista capable of running everything you need?
5. What undiscovered glitches exist in vista? XP is still releasing security patches, updates, etc.
6. Whats the rush? If it runs OK, fine. If not, you can decide to install XP at any time if you have a copy. (Thats just me, conservative approach, I have a copy of windows 3.1

.)
I have a copy of XP Pro in a box, just in case the next windows release is a clunker, or I want to build my own computer. Your post made me think about it, thanks. Will probably get another copy before the windows guys come to all the stores and get them like they did for W98.