i did use the same 651's in the back but that took some modified mounts to make it work right and i would recommend doing the same to anyone..
if you want a decent soundstage that wont blow your windows out then i'd get the following:
- aftermarket HU (pioneer, alpine and kenwood would be my brand picks)
- component speakers up front (separate tweeter and woofer. mount tweeter in the factory door pillar location to bring the sound up to ear level)
- 3 way/ 2 way speakers at the back with a flat face (no protruding tweeter cones)
- an amplifier to power the speakers. (i honestly dont have one since i dont listen at an ear shattering volume level so there's an option for you too :biggrin

- subwoofer. search the forum for an underseat one (go aftermarket and dont get an oem subwoofer). search "sea-scooby sub kit" too as he sells a complete custom kit.
- sound deadening. search "project quiet car" in the forum and read it up. a sound deadened car makes a big difference.
you could go slowly with the whole process too like what i'm doing (audio is NOT a cheap project at all). i started with oem tweeter kit> aftermarket speakers > underseat powered subwwofer (boss 8") with stock headunit and soundeadeners soon and the 3 upgrades already made a HUGE difference in sound quiality.
have you checked out crutchfield.com? that site has tons of info for anyone looking to upgrade their system.