Antialiasing quality
To illustrate the effects of antialiasing, I've run a "diff" operation between each card's original output and 4X antialiased output.

Difference between no AA and 4X AA - Radeon 9800 XT

Difference between no AA and 4X AA - GeForce 6800 Ultra

Difference between no AA and 4X AA - GeForce FX 5950 Ultra

The GeForce 6800 Ultra clearly "touches" more edge pixels than its predecessor. The edges outlines are more pronounced in the GeForce 6800 Ultra image. However, the Radeon 9800 XT touches more pixels than either—though not necessarily more edge pixels. The Radeon appears to modify more of the screen than the GeForce 6800 Ultra, which is probably less efficient.