Zapco makes an excellent line of entry-level amps with the iForce label. I'm using the Zapco iForce 450 right now which is absolutely fantastic for my purposes. The 1st two channels run the fronts and i'm bridging the 3&4 channels for the sub. For your configuration you could easily use the 3/4 channels to run the rears. I'm not quite sure if this is what you're looking for, but if so, I'd highly recommend it.
I believe they're going for under $200 on various places online. If you have a hard time finding one, try the guys at www.indoaudio.com/ (their phone and email are in their profile somewhere). Thats where I got mine.