stillrunning, I do car stereo installs on the side. First off, any aftermarket head unit is going to be tons better than a factory unit. Besides, they tend to be cheaper too.
As for new speakers, that is a yes and no answer. Do you need speakers? No, not really assuming that the factory ones have not been blow. But, at the same time, the factory speakers are a very little wattage speaker. So, if you get a new head unit with a high power amp (normally they are 50x4), the need for new speakers will be necessary. Along those same lines, those speakers are 15 years old and like anything else with time, they wear out and bigger and better models are released. So, going with new speakers (even if they are 6.5" speakers) will improve the sound you are getting by themselves.
Another thing that I will recommend, especially if you are looking at getting a higher wattage unit is to upgrade the wiring between the head unit and the speakers to something aftermarket. The factory wiring is barely adequate for the low power of the factory system. Also, on your 97, there is an integrated amp in the truck. While you can use an aftermarket head unit through it, the sound quality of that unit is not going to be near what a new unit is going to be.
If you need any additional assistance, let me know. I had karaoke in my Expy for awhile. So, to say I had a "nice" system is probably an understatement.
