I just upgraded my sound system for the fun of it over the course of a few months. wstefan20 is right. For simplicity, 5.25" coaxial speakers are going to be the best bet. The factory setup is component with separate tweeters and woofers, so a coaxial won't be the same. But just about any pair of coaxials should be a step up from the factory stuff. Especially 20 year-old factory stuff.
I'd recommend looking for coaxial 5.25" speakers that will work within your budget. I put in some 5.25" Morel Tempo Ultra coaxials for my front speakers and I am very happy with them. Just bolt in and go really. I ran them off of my aftermarket head unit for a few weeks before adding an amp and they worked fine like that. Morel has some more affordable speakers (i.e., Maximo) as well, but any decent brand should be a noticeable improvement.
