First...I'd like to ask what is "RMS Watts"? Why are speakers the only technology that really cultivates this measurement(if you want to call it a measurement...)? If you can answer that...your next question is what Matt asked:
"What are your listening habits?"
If you don't listen to sine waves you'd probably be fine with a "200 watt/rms" amp. Box design can be crucial but if you don't want to deal with that just go with manufacturer recommendations and probably 500watts or so.
Car Audio can get really scientific and out of control real quick. So if you're just starting keep it simple for the time being
SSD Enclosure Recommendations - SSA Car Audio Forum
If you go sealed make sure you choose the "high QTS" option. Otherwise just go with what they're promoting in the thread.
