You could always make some adapter plates out of 1/2" mdf and run 6.5's. That would open your options up a little more.
Look for speakers with a higher db rating, they will be more efficient with H/U power and be a little louder since you don't want to run an amp. Worry more about the speakers in the front then in the rear since the rear will mostly be just fill. Hell, just replace the front speakers first and disconnect the rears and see if you like it that way.
Best thing to do is go out to your local stores and listen to everything. It won't be optimum listening environments but it will be better then nothing. Also why you are at the stores listening, take a look around for open box and clearence speakers. I have gotten some pretty good deals that way.
