Lots of good suggestions here, but I think you guys are making it too complicated and expensive.
1. Buy this reciever yamaha rx-v663 ~350$
2. Set the audio selector to 7 channel
3. You now have evenly distributed sound from up to 7 speakers.
4. Profit!
For 900sq feet I can't see needing more power/speakers than that.
If needed you could use up to 9 speakers, but I really think that would be overkill for such a small place.
For a sub check out the dayton subs here on AH. Good bang for the buck and the free extended warrenty might be helpful in the retail setting.