The first thing you need to do is find out what the MAX RMS wattage your speakers can handle. Then look for an amp that is right there at that wattage(make sure you are looking at "RMS"ratings) it is even ok to go over the max of the speakers a little!! Now, in the post above Steve talks about running two speakers off each channel(that is ok) just know that will be running them at 2 ohms(make sure you look at the specs of the amp and see what the power rating is at 2 ohms, it will usually be doubled).
Post the specs of ALL your speakers and we might be able to show you which amp would help the best. This place has some good deals on amps. I have used them many times and NEER had a single issue. http://www.sonicelectronix.com/cat_25.html
I kinda like the Hifonics amps for a boat.