I'd actually suggest, for your price range, to spend the money on a two-channel setup rather than trying for 4 channels. You'll get far more clean power for that cash in a two channel amp, and far and away better speakers for same. Your front sound stage is more important from a sound quality perspective than any other part of your system, and having two good quality speakers is going to do you far better than four bargain bin drivers on a bargain bin four-channel. I'll put it this way: For the same money in your price range, even from the back seat, a properly installed two-channel system is going to sound better (or play louder clean, if that is your preference) than a four.
Check out 6.5" component sets from Pioneer, etc., and take a look at the reviews on DIY car audio forums on various speakers.