You sure have a lot of cars. :happysad:
But anyway. Generally speaking, the stock speakers arent great speakers at all, so swapping them out will give you a nice improvement. However, you have to realize that you only have a certain amount of power to work with using the stock headunit. Have you thought about adding an amp to feed all the speakers a bit more power? Have you swapped out the fronts yet? Getting a good set of fronts will give you much better sound because you are much closer to those speakers and they play the part of imaging with your system.
If you can, I would get a 4 ch amp and swap all the stock speakers out.
(Off topic, but with the website that you linked, how did you get the pictures onto the side of the page like that? That is something that I want to do to my webpage so that you can scroll down the picture thumbnails independantly of the rest of the page. Thanks
h: )