I suggest you upgrade your front stage to some decent components. The PG RSd 6.5s are VERY good for the money if they fit!
The only problem I see is doing it for $600 with new equipment. I did get a pair of JL 10w3v2s and a Rockford Fosgate P5002 from a friend of mine for $180 total out of pocket. If you look around, there are deals to be had. Getting great sound while spending $500 to $600 on new equipment is going to be TOUGH!
Then again, let me try:
Phoenix Gold RSd 6.5s = $60
Single 8" Sundown subwoofer = $60
Cerwin Vega EXL-400.4 - $150
Clarion DXZ785USB HU - $210
So there you have it, $480 out of pocket before shipping, wiring, and a box.
Changes I would recommend would be that you go with a single Memphis Power Reference 10 in a Memphis pre-fab box. That would add to the budget, but would take care of the sub and box.
As for the amp, run channels 1 & 2 on your front stage, and bridge Channels 3 & 4 for your rear stage. This setup won't turn heads from blocks away, but will sound GREAT to you with the right tuning!