you can probably find a really nice pair of Midiland S4 8200's for around $200, I got mine for $220.. also, then you'll want to get another cable for them, depending on your soundcard
if you have a sound card w/ 4 channel analog out (I.E., front and rear) then get the Intellicable from Midiland
if you have a sound card w/ 6 channel analog out (I.E., front, rear, and center/lfe, like the Philips Acoustic Edge) then get the custom cable from iec.net as it allows you to plug in a seperate RCA cable for each channel, then just go buy 3 mini to rca cables from radio shack or even WalMart
This cable is needed if you want to get all of the speakers to sound whenever you're listening to something not AC3 related, like playing games, mp3's, etc... whenever you watching a DVD or something like that, use the decoder box with S/PDIF simply because digital sounds so much nicer
GREAT speaker set up, make sure you get one of these two cables (again, it depends on your soundcard, as to which one you should get) or you may be disappointed from only getting sound out of two speakers.