As far as I know the only benefit of Vista 64 to gaming is the ability to use a lot of ram, some games run better and load faster when you have 4 or more gig of ram and 64 bit is the only way to use that much ram. This would be power hungry games, not peggle, but which games exactly I have forgotten. WIC and Crysis Warhead performed significantly better on 6 gig than 3 gig in this Corsair study: http://www.maximumpc.com/article/news/corsairs_internal_benchmarks_why_6gb_better_3gb
This is on a core i7 platform.
Other than that gaming is the same on any current MS OS at this time as far as I can see. Vista has caught up with XP on frame rates due to better video card drivers. Vista 64 is just a good choice overall for a gamer/enthusiast who doesn't need perfect legacy support for old apps and hardware - support is closing in on 100% as more and more 64 is becoming mainstream. Contrary to the Mac vs. PC ads Vista is not slow or buggy or whatever vague assertion they are making, it runs quite well now.