Antialiasing quality

6X AA - Radeon 9800 XT

6X AA - Radeon X800 XT

8X AA - GeForce 6800 Ultra

6xS AA - GeForce FX 5950 Ultra

8X AA - GeForce FX 5950 Ultra

Here the difference becomes more apparent. Although NVIDIA's new 8xS mode looks very good, ATI's 6X mode is more effective than NVIDIA's 8X mode, thanks to gamma-correct blends. Since ATI's 6X mode also performs better, that's quite the advantage. Add in temporal AA, and ATI has a pronounced lead in antialiasing overall right now.

However, I should mention that NVIDIA's 8xS mode does do more than just edge AA, and there are image quality benefits to supersampling that likely won't show up in a still screenshot.