nVidia is always one step ahead of ATI in performance. If you're planning to do some hard-core gaming, look into the 8800GTS/GTX/Ultra. The 8500 and 8600 is only for web surfing, photoshopping, photo-editing, video editing, encoding, decoding, and etc.
or X2900. I prefer the 8800, but the X2900 is no slouch. It will often beat the 8800 GTS's and sometimes the GTX's
One of the things I don't like is its performance with anti-aliasing
The 2900 is a great card, and it continues to improve as the drivers improve. Seems neither NVIDIA or ATI can get their drivers right on release anymore.