I second the fact that you probably don't need a center channel. Look for a used or old stock 5.1 receiver (Marantz, Denon or Sony) and a decent subwoofer. I don't know what your current room is like and how you would wire the back speakers but one solution is to do in ceiling speakers. Cambridge Soundworks sells their B-Stock speakers on ebay at a very reasonable price w full warranty's. I don't recommend them for the front speakers but for surround and in wall I think they're a good value.
If you want to go really cheap, just get a 2 channel receiver, 2 front speakers and a sub and you'll be 80% there.
If you want to go really cheap, just get a 2 channel receiver, 2 front speakers and a sub and you'll be 80% there.