http://www.audioholics.com/buying-guides/system-buying-guides/the-1000-bare-bones-system
Also consider the Impression 5.1 (
http://www.emptek.com/theater.php) and an inexpensive receiver from ebay with 5.1 inputs, then use the computer for decoding. (a little over budget).
Or my fav: 3 pairs of Behringer 2030p ($133x3 = $399) + 2 Dayton 10" subs ($150x2 = $300), plus a Behringer CX crossover ($70) = $770: leaving you $230 for a 5.1 AVR with pre-outs and an amplifier (unless your sound card, like mine, will output 5 RCA: in which case just get a 5-channel amp). That would, in my opinion, be the besst sound. In a budget crunch: you could hold off on one Dayton, the CX and (if you get an AVR) the external 2-channel amp (all these go together) putting you easily in budget.
Or one of a hundred other options. Search this forum and you'll find scores of recommendations for exactly this in the last 45 days.