Good idea. They (Crutchfield) are higher than some of the other online shops, but right on par with anything you normally find locally. They throw in extra cables and adapters if needed, give you a pretty good guide on stereo installation, and charge a good shipping rate on anything big.

Looks like any straightforward 4 channel amp should work, I'd look in the 50Wx4 range or so. You might make sure you have a built in high & low pass crossover for each set of speakers, so that you can cut some of the bass off the front set (might get a little louder before any distortion hits).

I've got Rockford Fosgate in my car and love it. I'm also partial to the Alpine amps, they are good quality and the newer style with the flip-up aluminum lid simply look nice.