With amps, bigger can be better because working a 200 watt amp at 50% load will give a much cleaner sound than running a 100 watt amp at 100%. And a cleaner amp signal is less likely to damage your speakers...

That being said, as far as I know, amps are not car or headunit specific, so compatibility won't be an issue. I think it comes down to power/quality vs. price. And it'd probably be best to get a 4-channel amp to take advantage of the RioCar's 2 sets of RCA outputs. After that, opinions differ and things get really subjective.

I have an MTX 546 4-channel amp that I just picked up "Open Box" at Circuit City for just over $200 (about 1/2 off retail) and I'm very, very happy with the sound quality over my previous Rockford Fosgate amp.