most 5X7s wont need an amp. but they will sound better with one. but if you are going to run them off the headunit, then get efficient speakers that can handle more power if you ever plan on upgrading. like brad mentioned, Audiobahn speakers sound really good off the headunit. and they can also handle over 100 watts RMS.
when you look at speakers, look at the rms rating. peak rating is useless.