We used the full version of Counter-Strike:Source for our testing. All in game settings were set to their maximum apart from anistoropic filtering and anti-aliasing, which we varied for different portions of our testing.
Our benchmark is based on a custom demo recorded using the cs_militia map. There are a lot of trees on this map so it makes heavy use of transparency anti-aliasing when this feature is turned on.
We ran our tests twice, once with transparency anti-aliasing and once without to see how this advanced AA technique affected performance. The lighter bars represent our results with it turned on.
The slightly more advanced pixel shading techniques and use of HDR in this engine really highlight the benefits of the 8600 cards with them consistently beating the previous generation cards by some margin. Both cards maintain playable framerates at up to 1,600 x 1,200 2xAA 4xAF and only the highest tested settings proved truly troublesome. Again, the Fatal1ty card is just behind the 8600 GTS.
As expected, the large number of trees used in our demo combined with transparency antialiasing makes for a big performance hit when this technique is enabled. However, I'd try dropping other in game settings before turning this off as the game looks so much better with it on.