ok sounds like you arent looking for anything crazy so my advice is to visit your local best buy or circuit city. usually you can listen to all the speakers they have and pick some that sound nice and are a decent price. most of the time they will offer free installation for car audio stuff that you buy from them. i suggest replacing all your speakers at once so theyll work together for nice even sound. i think it was mentioned that you probably have size 6 1/2" ones up front and 6" x 9" in the back. but you can double check on
www.crutchfield.com to make sure for your year and model. i also recommend getting a new head unit at some point and together all of this will sound much better than your stock system did. same deal with that i suggest going to the store and playing around with the display ones to see which one has a button layout that you like and functions that you want.
i say goto the stores because you can probably get free installs on most of these things and save yourself some trouble. if your comfortable installing things yourself then go check em out at the store and buy em online somewhere. ive bought audiobahn stuff off ebay very cheap and everything has been great (make sure you buy from an ebay store though)