It all depends on how much money you want to spend and what type of sound you are looking for! My own preference is for fidelity over sound pressure levels. For my own car, I use a crossover between the headunit and my amps. I have a separate amp for lows, mids, and highs. I use a powerful Rock/Fos amp for the low frequencies; not so much for high SPLs but for tight and clean bass at all volume levels. My mids and highs are driven by 2 separate Alpine V12 amps, the larger powering the midrange drivers. I also have four 1 Farad capacitors in parallel across my power rails to ensure the amps aren't starved for electricity during a loud transient.
Other systems I have built for friends, for the most part, are less complex than mine. Most can get by with one good 5 channel amplifier which has crossovers built in. Some want redonculous amounts of bass and could care less how their setup sounds on the whole.
Let us know what you want out of your system can we can give you a better idea of how to set it up properly for your application.
-Bill