If for some reason stereo audio doesn't satisfy your gaming needs, you can get a 3D audio card. This basically adds surround sound to your computer. You'll also need new speakers. If you don't know why you would want this, you don't need it.
Right now, there are no 3D audio cards for Macintosh that I know of. If any are released, I will post it here. For now you can still get speakers: