Personally, I think nVidia cards are fab Kimo, I've used them for ages now and will in all probability continue to do so into the future. As far as I'm concerned, I've always found them to be really good - fast, reliable and reasonably straight-forward. The lack of issues with drivers is also a big plus - each time a new one comes out, I chuck it onto the machine and everything runs as it should. Windows 7 even handles the download for me...doddle.
Only slight downside is the fact that they do run quite hot so you need to consider the cooling, but that may well be the same for ATI.
Jabo