if you have enough power you can get most games to run on either platform, you have to do a little bending the rules as vista often wont let some parts of programs read certain other parts of data etc. but i managed to get all my games running in some sort of fashion on vista. it's a damn lot easier to get vista games to run ob xp though.
i think sp1 will address this as vista struggles, with 3d programs such as games atm with the way video ram is allocated.
best thing to do before you buy a games is to go on the website forum and find the code and tweeks needed.