Counter-Strike: Source beta Video Stress Test
The Counter-Strike: Source beta includes a video stress test that's essentially the same test Valve used for benchmarking Half-Life 2 performance last year. The video stress test presents a sort of worst-case scenario for graphics in the Source engine, layering surfaces with multiple pixel shader effects on top of each other. If a card does well in the video stress test, you can probably expect it to run Half-Life 2 quite well.

The 6600 GT runs with the big dawgs once more in the video stress test, until we hit 1280x1024 with 4X AA and 8X aniso, where it drops back to run with the 5900 XT. I'm curious to see whether a 256MB version of the 6600 GT would avoid at least some of this performance penalty at higher resolutions.