ATI's Drivers are excellent these days. Nvidia's I can't say for sure, but I have recently went from a X800XL to 7900GS. I have some beefs with the 7900GS, but haven't spent a lot of time trying to fix the problem I'm having with it, so it might not be Driver related. At any rate, the situation is kinda funny so here it is:
I do some light 3D Modeling as a Hobby with Maya. For years when someone asked about a Video card for Maya everyone always recommends Nvidia over ATI due to some "ATI issues". When I started using Maya I had a 9800XT and back then there were some glitches withit while using Maya. Not major issues, just some minor annoyances. After upgrading fom WinXP to WinXP 64 and possibly from my 9800XT to x800xl the glitches just went away and Maya has been working perfectly for a couple years.
A few weeks ago, a friend upgraded his 7900GS to a 8800GT and gavve me the 7900GS. Though it's Performance is approx equal with the x800xl, it does havve SM 3.0 which fixed some iissues with UT3 Editor, so I was enthused with the idea. So I uninstall the ATI Drivers/Utils, take out card, install 7900GS, Install latest WinXP 64 WHQL(required for XP 64) Nvidia Drivers everything was swell. UT3's Editor ran without Error/glitches, games were fine, yaddayaddaya. Day later I fire up Maya and begin working on something, a few minutes pass when all of a sudden Maya freaks out on me. I was not impressed one bit and I quickly began thinking of putting the x800xl back in. I sorta of stumble upon a temporary solution(simply moving the mouse pointer over the Auto-Hide Windows Taskbar ends the glitch in Maya....until it occurs again. So, that's what I've been doing for the last couple weeks while using Maya. It's a PITA and I still swear a lot everytime it happens, but at least I can work.
Things I might try but haven't: 1) Driver cleaner...maybe there's some ATI stuff hanging around; 2) older Driver version, these Drivers are already many months old; 3) re-install Windows, no thanks, suppose I could Repair, but I'll hold off