You've got to compare apples to apples. Use the RMS wattage for all comparisons.
In sizing an add on amplifier, simply put, you're looking for an amp that is rated between 100-150% RMS output on a single channel of the lowest RMS rating of a single speaker in your system. The smallest speaker in your system rated at 100 watts RMS? Get an amp that has 100-150 watts RMS output per channel.
You do not add the wattage of your head unit to that of the amp. The amplifier is just that. It amplifies the audio voltage, but it can't output more than its rating.
What you don't want is to have to crank the head unit all the way up to get the desired loudness. That's the point where your head unit is putting out its distortion rating, and the amplifier will just make that worse. That's what the amp's gain control is for. You may find that you'll spend nearly as much time setting the controls of the amp as you do installing the amp.
I've found that Car Audio Forum is a resource that quite compliments Crutchfield's website.
