What should I consider when buying an amp for my car?

There is a lot to look at when buying amps. First, rule out any brand that isn't CEA complaint. There are a ton of junk brand amps out there that will promise you inflated wattage that will never be achieved. Boss, Power Acoustik, Lanzar, Legacy, Dual, Pyle.. the list goes on and one. Rule those out. Then, dont' just look at power, although you should be looking for an amp that will match the RMS rating on your subs at whichever impedance you will be running at. Look at THD, or total harmonic distortion levels, the lower the number the better. Also look at signal to noise ratio.. the higher the better. My advice is to take your sweet time when choosing an amp, as it is the most important part of your system. Alpine, Kenwood, and Polk make entry level amps around $200 that are efficient, and will provide around 500 watts RMS power out of one channel. I personally like JL the best, but they are not cheap. Just be careful at the 'hock shop'.. I frequent pawn shops a lot, and its mostly just junk brands like those mentioned above. YOu might find a nice old Rockford in there though. Just take your time, and make an educated purchase.