IMO, I don't see why any more than 3.1 or even 2.1 is needed. Five speakers and a subwoofer just seems to me like a lot of unneeded clutter and wasted money.
If you really wanted 5.1, I think it'd be just as well to get a 5.1 surround system with the reciever, and connect your computer to that. I did that with mine. You'd get a lot more uses out of it, and I doubt it cost much more, if at all.