Get Windows Vista in preference to XP. By now, most applications are optimised enough to run on Vista as good or better when compared to performance on XP.
And don't mind if results are published for Vista 64bit. If they really ARE, go get vista 64bit
The only propable problem with vista is that its interface takes a LOT of time getting used to. Several things are hidden away in odd corners and so many things which you could do with great ease in XP would seem damn hard on vista in the first week or two. You may have to use a few tuner software to optimise the OS. Personally speaking, I got sick of it in a day or two, but I am sure you have enough patience to spare on this OS.
The reason I am telling you to get vista is because WIndows 7 is to Windows Vista is to Windows XP what Windows XP is to Windows ME is to Windows 98. When Windows 7 comes out, there would be much better performance but much less to learn.
The monetary investment on Vista is half that on XP for a retail license. But that shouldn't worry you should it ?

Again reason to get vista.