You can do as I did... Hook up some nice hifi speakers to your pc by using a converter plug, they sound way better than any stereo speaker can for that price, plus, if you already own them, they're free.
I have two hifi speakers and I bought a Logitech X-230 2.1 set with it (same quality as the X-530 but it's the two speaker edition I've been told) to hook them up as rear speakers. I can tell my hifi stereo speakers sound much better than my Logitech pair, then again if you use 'em for rear speakers you don't need the greatest sound there.
If you haven't got any speakers at all and you really want surround, do go for a 5.1 set, like that X-530 or X-540. But I'll warn you, the bass on the *****es is LOUD.