Cross-post from Electronics forum. I did a fairly extensive write-up over on AudiWorld.com regarding replacing stock speakers all the way around and adding a 4 channel amp:

http://forums.audiworld.com/audio/msgs/52347.phtml

heh...
almost going the same route but i will be replacing the whole system.
Headunit - Kenwood KVT-717DVD
Wiring By Street wires ( 4Gauge for power and ZN60 for signal)
Amp - Memphis Belle 16-MCH600
Front and Rear Speakers - Memphis 15-MSYNC6
Subwoofer - JL 12W6V2
Subwoofer in custom fiberglass enclosure in the rear right side of the trunk.