First up without any clue what size room (in cubic feet) it's hard to even hazard a guess sizing a sub. But I'll take a swing at this. Three pair of Behringer B2030P studio monitors for $131/pr from B&H Photo (
review) or Amazon. That will give you 5.0 (plus a spare) for $400. Add a $155 Dayton subwoofer (or 2) from Parts Express and you have a decent small room (w/1 x Dayton sub) 5.1 system for around $600.
Or you could buy 2 Boston Acoustics CS26 bookshelf speakers for $80ea (Google is your friend), a CS225 center for around $150, and a pair of Wharfedale WH2 surrounds for $80/pr and have 5.0 system for $400 again. Add the $155 Dayton sub (or 2) and you have a small room 5.1 system for $600. Or substitute 2 more CS26 bookshelves for the Whafedales and have a pretty decent 5.1 system for $700.
Or you could start with a single pair of say the Behringer B2030P speakers and a better sub (
eD A2-300 or a
Hsu Research VTF-2) for a really good 2.1 system that will do well in a medium sized room depending on which sub you choose. This is the route I'd probably take (with the VTF-2). Then add more pairs of 2030Ps down the road as money allows. Budget another $50 for an Auralex SubDude or the larger Gramma to put the sub on. The B2030Ps aren't pretty but sound really good right out of the box, and really and truly rock after a couple of DIY mods developed by WmAx.
All prices are USA and you'll have to add in additional shipping and duties.