Mike,
This forum is very helpful when it comes to these types of things. I had the same quesiton and was led to purchase 4 Infinity 602m's, got them off e-bay for $73.00 a pair and Joseph had a link for a 4 channel 600w Pioneer amp that I picked up for $129.00.
I put the other two speakers under the rear bench, cut the carpet out where the predrilled holes were and installed them myself. I did take the boat to Circuit City for the amp install and the final wiring for all 4 speakers. Needless to say, my stereo now kicks some major butt. My next move is a dual battery with a sub and another amp, but that's gonna have to wait till next season. Need to use my money to go enjoy the boat now. Just got back from Havasu and the stereo proved to be well worth the money.
Garth Bachman