The rear speakers are just woofers... no tweeters.

The simplest way is to get new rear speakers (a nice 2 way set) and new front components... then just mount it all in the stock locations. Something from Polk Audio or Boston Acoustics will probably give you goosebumps.

If you go with higher end speakers (more expensive) and add a strong 4 channel amp you wont need a subwoofer most likely. A good pair of 6x9's will produce a pretty good amount of bass when amplified and tuned properly. Something with 75 to 100 watts RMS per channel would be good.

Just adding new speakers probably wont please you... adding amplification will bring out the speakers detail and allow them to play at higher volumes without distortion.

With the front speakers, you shouldnt have a problem fitting new ones in there... there is a good amount of space between the speaker and the grille, so if a small spacer has to be used to gain depth, it should be ok, and nothing will stick out.

Good luck with it!