I believe it depends on the game. It usually says on the back. I think God of War 3 is in 1080i but most games run at 720p. The PS3 supports 1080p playback and the 360 supports 1080i I believe. correct me if I'm wrong.
most games run at 720p natively.
there are VERY FEW games that run at 1080p. if i remember right, a couple of tennis games or something ran 1080p lol.