An external amp would make a great difference to whatever set of speakers you want to power, whether the amp is small or big as long as you are powering the speakers properly.
Eclipse and Infinity Kappa components would sound nice with them, maybe even CDT if you have the money.
For headunits, look into Eclipse, Alpine, Pioneer Premier, and Clarion ProAudio. Be sure that the headunit has Front and Subwoofer RCA outputs.