Ive only tried the beta but it does run a lot better on XP.
Im really underwelmed with vista (x64) for gameing. Every game I have tried there is a fairly significant drop in frames, and there is no difference in IQ ( I know someones going to say DX10 is for better performance over DX9 not an improved visual quality but it doesnt even do that at the moment)
I only run it at 1600x1200 but that is more than enough for me for my 21" CRT.
Heres hoping the service pack for Vista improves things, but from memory Im sure XP had pretty good gameing performance compared to 98 from the start (not sure though)