I've got a 7900GS and I am currently testing a few games using the default drivers of Vista Home Premium (v 7.14.10.9686).

So far I tried:
- Splinter Cell Double Agent Demo: unplayable due to constant flickering (looks like a Z-Buffer issue) in the game. Menus and cut-scenes seem ok.
- Neverwinter Nights 2 with current patch: unplayable due to flickering and display errors even within the menus.
- Flight Simulator X: Actually seems to run better on Vista than on XP. I didn't do any benchmarks, but it feels more fluid and has less graphical errors.

Overall though I hope that the problems are only due to the NVidia drivers. Otherwise Microsoft will have a hard time convincing gamers to switch to Vista if only games specifically designed for Vista will work properly. Currently I would not recommend to switch to Vista if you want to continue playing games. Maybe dual-boot at first to see what works and what doesn't.