I will give this a try, although I am not an expert by any stretch of the imagination.
I have an XBOX and as such am a little bias. To my knowledge it is the only one of the three (XBOX, PS2, Gamecube) which does 5.1. (Not 100% on that one) It also has the "Capability" to play games in 720P and 1080i. I say capability because I don't think the library of those types of games is that extensive. Some sports games are I believe and I hear they are impressive. Even Halo2 which just came out is 480P. (There are numerous threads on AVS about the 480P/Widescreen problem with HALO2).
To get 5.1 you need to get either the Component or S-Video(I think S-Video has an optical slot) AV pack and an Optical Cable. You automatically get the composite AV with the XBOX.
