I'm planning on getting a 50x4 RMS amp. The stuff is pricy on crutchfield though so I'm thinking of getting stuff on ebay/amazon. New in box of course.
Watch ebay, sometimes you get refurbs. I was got really good one time on an Audiobahn amp.
You could also do a 3 channel amp, as stereo is 2 channels and who really uses the fader.... tie the front and back together and run them as a 2 ohm load, then have the other output for a sub later on.
I'd personally ditch that headunit and for the money you're putting in an amp, you can get a NICE in dash DVD that has 50x4