Component sets are a woofer and a tweeter with a seperate crossover to direct higher frequencies to the tweeter and lower frequencies to the woofer. they tend to cost more as well. i would suggest a midbass driver style speaker, which is basically the woofer from a component set, or getting similar style speakers to what you have in the front and adding a 4channel amp.

another option of course would be to get a 4channel amp, power your fronts, and find a nice 8-10" woofer and run it off of the other 2 channels of the amp.