I don't game, so, I can't say from first hand experience (nor have I read up on it as it doesn't affect me), however, coming from an observational stand-point, I seem to notice more issues along the lines performance and compatibility (drivers etc) with Vista than XP on this board. In more than one thread, playing games such as Crysis with DX10 on Vista resulted in pretty lousy frame rates. I'm not sure how wide-spread that is, though.
Personally, I'd probably use XP at the moment if I already owned a copy of it. If I were going to pay for an OS, I'd buy Vista, because I assume at some point down the road, kinks will be worked out and things will be refined, as they usually are with operating systems. Plus, there will obviously be improvements in hardware, so Vista/DX10 won't seem as heavy.