I replaced my rear speakers and the difference is amazing. Much better! The problem is that the fronts can't handle any volume, so I have it slightly faded out in the front so bass does not distort it. I have a 4" component set lying around that I plan on making a plate for to install the 4" and the tweet. That should take care of it, and the total cost is about $300 (from crutchfield about 4 years ago).
Crutchfield is a good place to deal with, I recommend them. Prices are ok, but their help and tech support is great. They will include wire harnesses for free with speaker purchases, so you do not have to cut factory wiring.