Depends on the game, but most games run at 720P and upscaled to 1080P.
BF3, is 720P with 30FPS, Crysis 2 is 720P at 30FPS as well. Unreal 3 games are the same as well.
Only a couple like CoD run at odd ball resolution like 576P at 60 fps. Alan Wake, and Halo ran sub 720P as well.