The R420 (X800) was just a fancy version of the 9800. It would have outperformed the 6800, but nVidia sacrificed so much image quality for performance to keep them in the lead. If you look at the X800 vs 6800 screenshots in various games - You will see how much better quality the ATi produces - So much, it is almost a joke! One of the most noticable is in FarCry. ATi never drops image quality. It only increases.
When they release the R520 GPU - Say goodbye to nVidia.