My 9300 is on vista. Everything runs fine. The sound driver and a few other drivers needs to be installed through windows update. The subwoofer even works with microsoft's drivers! Performance wise, I think vista is solid. Not extremely fast, but I would say it is comparable to xp except the video driver. I would stick with the video driver vista installs for you. It seems stable. Nvidia has horrible support for vista at the moment.
In fact, after I installed the new vista drivers listed earlier on this post, my screen would not turn on after long period of time. I have to go into standby to log in again to have the screen turn on. I do have a second monitor plugged in, so maybe that is the problem?