Halo 3 runs at a lower resolution, not only to maintain a constant frame rate, but because it runs TWO frame buffers, that cover both the high dynamic range, and the low dynamic range. Hence why Halo 3 is possibly the most colourful game in a long time.
80 vertical pixels is a worthy sacrifice to not have a game that looks bland like Gears of Grey, and to have a steady frame rate during some of the larger battles (at one point in the game, there are 2 scarabs, about 8 ghosts, 20-ish brutes and grunts, 10 or 15 marines, and a buttload of aircraft having dog fights in the air. All in an area that is probably about 1/2 mile wide)







