I currently have Vista Ultimate. There are practically no problems with it. What I've noticed is that when you clean the registry, there's a huge number of broken keys on XP, while Vista has far fewer registry errors. By the way, Vista has optimized the performance of applications designed for multi-core processors, especially games, but Vista still consumes a lot of RAM even with the standard classic theme and a minimum set of services, so the FPS in games is still higher on XP.
[Offtop_mode_on]Mass Effect doesn't work on XP for some reason; I'm playing it on Vista...[Offtop_mode_off]