[scoobasteve]

It really wouldnt be worth it to upgrade to an audiophile setup.

You can add a sub to the factory setup, you'll just need a line output converter so you can get RCA connections off of the rear speakers. Or another option would be to buy an amp with High level inputs.

Both would be much easier than wiring up the factory sub and speakers

Personally, i'd hold off for a bit, Save some money and do a simple but effective overhaul on the cars stereo

I'd reccomend going with:

Any H/U that has 3 sets of preouts
5 Channel amp. Something that will do 30x4 and like 200x1
2 pairs of 5x7 coaxials.
and a 10" sub.

If you shop around, check car audio forums you should be able to get all this for under $1000.