To compare the performance of the 9800GT and 9600GT, I chose four DX10 games, Crysis, Company of Heroes: Opposing Fronts, Call of Juarez, and PT Boats: Knights of the Sea.
Crysis is a real resource hog, and besides a powerful CPU and lots of memory, it prefers a strong video card. The 9600GT has proven in the past that it can handle AA with Crysis, the first midrange card I've had that would. I used the Crysis Benchmark Test by Boris Verziga, settings were Medium, DX10, GPU Test, 2 x AA, 1680 x 1050.
Though the 9800GT did come out ahead, I think that the 9600GT's gig of memory helped it out here.
Company of Heroes: Opposing Fronts wasn't released as a DX10 game, it was added in a patch. It really doesn't act the same as other DX10 games, as you see a lot higher FPS. I used the onboard performance test, settings were: High, 2 x AA, 1680 x 1050.
PT Boats: Knights of the Sea is a water-based game with excellent graphics. The benchmark has some tough camera angle changes that really affects the low FPS. I used the standalone benchmark, settings were High, 2 x AA, 1680 x 1050.
Call of Juarez is an FPS in an old west setting, with incredible graphics. It really pushes a GPU, even the top video cards don't get really high FPS with this game. I used the standalone demo benchmark, settings were High, 2 x AA, 1680 x 1050.
The 9800GT just barely pulled it out here, obviously due to the 9600GTs overclock and greater memory.