simplest upgrade please!

i have 94 E320. recently my factory cd changer gave up on me. i went ahead and bought a sony cd changer 'cause the dealer wanted 600 bucks for a factory one. w/ the sony changer, i also installed a new sony deck in my dash, replacing the factory deck. i put in the sony cdx-m630 w/ motorized face plate and cd player.
the deck improved the sound a little bit and now i wanna do something to improve the speakers. i'm a starving college student! thus, i don't want to spend too much going crazy doing all those cool things everyone else in this forum seems to be doing!!! i want to simply upgrade the speakers in the car (the two in the front dash, the subs in the doors and the 2 in the rear) any suggestions? my main complaint is that the speakers in the doors suck. maybe change the door and front dash speakers? dunno...
the receiver i got gives off 23.2 watts per channel minimum continuous average power into 4 ohms, 4 channels driven from 20Hz to 20kHz (copied from manual!) can i just replace the speakers and use the audio outputs from the receiver, run new wires to speakers? and leave all the other wires there unused? any suggestions for speakers (and please include approximate cost). can i do it myself? i'd like good sounds w/ decent bass. for music from dave matthews, to linkin park, to nelly to dido, from santana to pink floyd... across the board music kinda guy...!
please remember, i'm staying as far away from cutting any metal or anything hard. i wanna just drop in speakers if possible. would doing this noticeably improve sound quality?

thanks!!!
joel