If you're using a stock head unit, honestly the biggest upgrade in sound you're going to get is to get a new head unit with a nice clean amp in it. I drove a '95 318 and man, that stereo sounded bad, but it was the lowest of the low end stereos that one could get with that car.
If you really don't want a new head unit and want new speakers, what will happen is that the head unit (if it's bad) will put out an unclean music signal, and the good speakers will reproduce that unclean music signal well and it won't sound that much different, however you may get a bit brighter on the high end because better tweeters.
I'm into sound quality and staging and imaging. Your stock speakers in the rear are probably more than adequate at providing rear fill, which is their purpose. The front speakers should be producing most of the sound and making it so you can actually see where the instruments are placed on your sound stage.
I have heard the alpine and mb quart component systems.
I liked the alpine, sounded pretty smooth and natural.
I have also heard the boston acoustics pro 6.5 and 5.25" speakers and I like these a lot. They are available at "Tweeter" but at true retail price and no discount = (.
The mb quart system had bright high end, if you're into that and everything sounded sibilant, but the midrange and midbass was very clean and transparent (this is a good thing).
People rave about focals, but I think they're going to sound more like the mb quart than the alpine.
What I would do if I were you is get a new head unit and a new set of component speakers for the front and just leave the ones in the back. The stock location for the speakers in the bmw is OK, but, there is nothing like the sound of properly set up kick panels. Maybe Q-forms makes some kick panels that would install fairly easily and inexpensively (custom work is expensive!).
I have heard also good things about CDT components (
http://www.thezeb.com) and infinity components. Really, there is no replacement for actually listening to the speakers you want to get and getting ones that you like.