Do you want loud or quality? Loud... just buy a big sub and amp. Boom....Thump...Boom

If you want good sound quality buy a set of components for the front doors, deaden the doors, amp to power the front doors, disconnect the rear speakers, any aftermarket HU, and small amp/box in the trunk. Head units just don't have the power to play loud without distortion. Don't forget to adjust your gains and tune your headunit to kill peaks and valleys.

$100 basic aftermarket headunit
$200 6.5 components
$100 install kit, wiring, fuses, mass loaded vinyl, etc
$100 front amp
$200 10" quality sub
$150 sub amp

Run a good set of amp'ed front components, disconnect the rear speakers (they will sound like crap and you won't be able to hear them over the fronts), and add a small sub if you want more thump. My 6.5 Image Dynamics hit harder than most cheap 8 inch subs. (Proper install is key!) They sound amazing! Seriously. I have not heard a better setup unless you jump up to professional car audio. They are clean, loud, and with excellent midbass. I spent a weekend deadening, sealing, and dampening my doors to ensure the speakers would perform to their full potential. If you want more (or deeper) bass than run a 10 or 12 inch quality box/speaker.

Here is my setup. I have one amp mounted under the front passenger seat and the other amp mounted upside down on the package tray.

Proper installation is key! Stick a small square of off-brand Dynomat on the center of each outer door skin. Remove the plastic sheets from each door and seal the inner door skin with off-brand Dynomat. Firmly mount the midbass speakers with wood. Tweeters fit in the stock location. Hang mass loaded vinyl on each inner door skin (glue along the top). Remove the chunks of foam from the rear doors. Stick some jute pad in the openings of the plastic panels and glue some to the back of the panels where it will fit (especially the rear doors where you have more room). Deaden the door panels, seal from the outside, and block noise with mass loaded vinyl. Makes the car noticeably quieter and sound system sound much better.