I don't know how many times I have to say this... I may as well put it in my sig.

REAR FILL IS WORTHLESS. There. Said it.

If you're looking at replacing anything, replace the fronts with a good set of 6.5 components. You'll need an amp and you'll be tearing into the dash to hook that into the deck... I would seriously consider looking into an aftermarket head unit if you're looking for any sort of upgraded sound quality. Upgrading speakers without adding any control to them is useless in my opinion.