I have always been told to pick an amp that has MORE rms capability than the speakers. This supposedly helps keep the amps from over heating because you are not running them as hard. I would go talk to a professional installer (not best buy or some big store, but a place that specializes in stereo installs). They will give you more information than what you want on what to buy, and how to set it up!
When you start adding amps, I do know that you MUST ground all of the stereo equipment to the same battery. My setup is from the factory, and when I listen to the radio I get engine noise. I need to check all of my grounds. IF you end up with this problem, and can't get the engine noise out, you can purchase electronic filters (they run about $20). Put these between the head outputs and the amp inputs, and ground them to the same battery as the stereo equipment. I had to do this on my last Alpine install and it worked great!
2007 Supra 24 SSV 340 CAT
Lake Bryan, TX