About the best that you get for close to that budget that have any chance of filling a room that size are 5x
Infinity P153 speakers that are on sale for $50ea at Newegg this weekend, plus an Elemental Designs
A2-300 subwoofer. That's a total of $665 including shipping.
My advice however is to start with a decent 2.1 system and add the other bits later. Start with two
P163 bookshelves which will perform better in that room and the eD subwoofer linked above. Then when you have more money add a PC251 center and a pair of P153 as surrounds.
Another good option would be to build a good 3.0 system using a pair of
P362 and a
PC350 center. Later when you have more money add the same sub and surrounds.
If you can double your budget I'd go with the P362/PC350/P153 plus a
Rythmik FV12 for a thoroughly butt kicking moderately priced 5.1 system.