Actually on better thought you probably could replace your car's speakers without too much trouble. Just go to
http://www.crutchfield.com/ look up speakers and punch in your car. The website will automatically look up what speakers will fit in your car. When you buy the speakers from the website they will even include custom intructions for your car on how to install the speakers! It really is the best place to get car audio stuff hands down.
There is also a trove of info on how to install amps and they have a large selection of powered subs. Im planning on getting the Kenwood KSC-SW1, it's small enough to fit under a seat and has decent sound...
anyway sorry if i have gone on too much.