i'd add a decent pair of 6x8 to the back, pack in a 4-channel amp. if you feel you want some decent bass, think about a hard hitting 10" (alpine type-r) or bigger louder SPL 12" type-r / rockford p3.
gotta obviously get an amp with those two subs but either one of them will work with the $210 mono alpine amp best buy sells (500w rms @ 2ohm).
basically if u want the better SQ, amps are a necessity. personally i dont give a crap about the back seats, i went 3-way components up front for a full soundstage (screw the rear fill!) but thats just me.
lemme know if you have any other questions, id be glad to answer them for you.!