2 12 Xplod's vs. 2 15 BMF's

Either go ported or sealed.  But since no one chimed in about your factory dvd system, yes you can add an aftermarket head unit and keep the screen in the rear working, i have done it.  You have a few options, 1. Replace your head unit with a nice unit with a AUX input and get a cheap dvd player to link your audio to your headunit and video to the screen. 2. Get a DVD head unit and just send the video to your factory screen.  You can also put some head phones in their so you can listen to your tunes on the head unit and passengers can watch and listen to their movie at the same time.  Many options.  #1 is the most economical costing approx $500 or less, or more depending on the quality of the head unit, but for this task i definately reccomend one of the Eclipse head units with the circle surround feature $400 cd8445, or $500 cd8455 with the digital input from the dvd player.

Personally i would do the headunit upgrade before changeing up the subs and amps, it will make your current system ( sony, i know ugg) sound much much better.

Basically any CDA model Alpine head unit is capable to do what you want, however the only models i reccomend would be The CDA-9830, CDA-9831, CDA-9835, CDA-9855, all the other models have pretty much sunk in quality beyond what i would call satisfactory.

Definately still check out the Eclipse head units, they are very compeditively priced, look at the CD5435 $350 or CD5425 $250 as a cheaper alternative but still with 3 pre outs and built in crossovers to protect your factory speakers from being demolished from bass until you can afford an upgrade, or desire an upgrade.