On my non dsp system, I initially just installed a sub in the rear. The door speakers sounded even duller than they ever did, so I knew I needed to replace them. I went ahead and just slapped in a set of MB quart components into the front doors and was very content with the overall sound inside the car. For kicks I also changed out the rear doors with another set of MB quarts and was that much happier.

IMO, you can get away with just replacing the front door speakers and having a great sounding system overall. Just pay attention to the amount of power the speakers want to be driven with. Getting a top of the line speaker might not be suitable for the output from the factory amps. If you want to avoid adding another amp for your mids and highs, keep it conservative.