I run four infinitys and I would highly recommend the alpine MRV-F340 four-channel amp. It's 55 watts x 4 and it really sounds incredible. After I properly set the gain to the front and back and upped the crossover frequency on the 6.5's in the front and the 6x9's in the back, I was able to crank the volume past the point of my own tolerance (really f'ing loud) and the sound was crisp and absolutely clean.
You shouldn't have to pay $400 for this amp either, I was able to find it on eBay from a private seller for $210 including shipping. It's really worth every dollar.