With surround sound, you will be able to actually hear things behind you. When properly set up, you will be able to pinpoint exactly where things are, even when they're not actually on the screen (IE: a car coming up to you from the rear driver's side will
actually be heard approaching from the left & behind). So yeah, it can definitely improve gameplay.
That said, stay away from those all in one HTiB systems. You will be much better off in the long run starting with a good receiver & decent speakers, then upgrading as you can. HTiB systems are basically throw aways.
I believe the 360 only supports 5.1 channels, but I could be wrong.