I have been buying nVidia cards, but I've been very displeased with their drivers (and that's stating it mildly). Their widescreen aspect-ratio scaling has been broken for years, and they do not seem to care.
With that said, I bought an HD4850 a few weeks ago, and ATI's scaling didn't work either. It was a beast of a card, but I had to send it back because it began overheating. It also sounded like a leafblower when the fan kicked in.
When the prices go down on the HD4850's, I'll probably buy another one with an aftermarket fan.