FARCRY i bet it will make your 6600GT cry when u install all the patches.
HALO is another game which can hit the card and check its endurance. ATi X800XT PE just gave 88 FPS in it while in quake 3 it gave above 420FPS.
well known Doom3.
Half Life2 is not stressing the GFX card but it does stresses ur CPU