Can someone explain the differnces between the Promedia Ultra 5.1 system and the Promedia GMX D-5.1 system? Why the $200 difference and why would I want one over the other? BTW, I would us them for pretty much everything: games(hardcore), movies(generally at least one or two a week), and music(all the time). I don't have a whole lot of money right now and the GMX are only $160 refurbished, is there any reason to hold out untill I can afford the Promedia Ultra?