ATI (in my opinion) currently has a stable driver platform especially for Vista. Although I believe Nvidia makes a superior product, ATI's performance comes very close and is in my favour for stable drivers ... for the time being.
nVidea, because they have better performance, and because their card is so configurable. The drivers are sweet, and you can run logic code on the GPU as if it were 8 more CPUs.
I've had both, and I believe the nVidia to be slightly better than the ATI performance-wise. They are both excellent cards.
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Ati is the best right now, they have the highest end graphic cards with the cheapest prices. I have one, and it's hasn't made a problem in two years! Go Ati!