Texture filtering quality
Once we dye the different mip maps colors using Serious Sam's built-in developer tools, we can see the difference between the filtering methods more clearly.

Trilinear filtering - Radeon 9800 XT

Trilinear filtering - Radeon X800 XT

Trilinear filtering - GeForce 6800 Ultra - 60.72 drivers - Optimizations disabled

Trilinear filtering - GeForce 6800 Ultra - 61.11 drivers - Optimizations disabled

Trilinear filtering - GeForce 6800 Ultra - 60.72 drivers - Optimizations enabled

Trilinear filtering - GeForce FX 5950 Ultra

Here are NVIDIA's trilinear optimizations at work. Mip-map boundary transitions aren't as smooth as they are on the Radeon X800 XT and on the GeForce 6800 Ultra with "brilinear" disabled. Notice the odd mixture of filtering going on in the 61.11 drivers with optimizations disabled. The floor and angled surface look like they should, but the wall's mip map boundaries show the dark banding indicative of the "brilinear" method.