I have not heard of any drivers from ATI for linux. As I understand the current situation do you not have any alternatives for consumer cards other than NVidias. At least not if you are going to do some “real” work.
NVidia is supposed to come out with a new card soon so probably is it best to wait for it.
Actually ATI has provided 3d accel drivers for the 8800 (fireGL) and the 8500. I’m not sure how they perform though, as I don’t have one. I did have an origional radeon, and I used a driver from xigraphics. The card worked very well with that driver.