If the OP can go to $600, one might consider this as a starting point:
2 x Polk Audio Monitor 50, FL/FR, $200 (http://www.newegg.com/Product/Produc...82E16882290013
2 X Polk Audio Monitor 30 Series II, SL/SR, $100 (http://www.newegg.com/Product/Produc...82E16882290200
1 x Polk Audio CS1, Center, $100 (http://www.newegg.com/Product/Produc...82E16882290210
1 x BIC F-12, Subwoofer, $182, (http://www.shopblt.com/cgi-bin/shop/...er_id=!ORDERID
Monitor 30 Series II and CS1 prices are good through 8/8 but IIRC, NewEgg does these sales every weekend.
BTW, that's a pretty large room which is the reason I noted this could be a starting point. For a room with that volume, a bigger/better subwoofer would be needed in the future to do it justice IMO. Also with a big room like that (depending on its configuration), the end game ultimately might be a 7.1/7.2 set-up.