Like the others said it depends on what you want out of it. Do you like to burn your own music, you would want to change out the HU and get one that plays MP3s. If the speakers aren't kicking out enough power, some guys have kept the HU and gotten amps and new speakers.
I just replaced everything on mine, because with what I wanted it was just easier. It seems a little intimidating, but it isn't really that hard at all, maybe a little time consuming, pulling everything out/off. It took me a weekend to get mine in, a new HU, component speakers in front, coaxials in back (most people just pull theres out and dont even use rear speakers), a cd changer and the amp.
Here are some helpful links:
A basic car audio site that has really good info, if you don't know much at all.
http://www.bcae1.com/
A car audio forum that can answer any question you could possibly come up with or if you want to read evaluations on systems or compenents.
http://forum.elitecaraudio.com/
A link that has links to on-line places that sell equipment. You can probably save at least 30% if you do it all yourself. Just make sure they have warranty's on the equipment you buy.
http://www.caraudio.com
A guy here on the forum sells the mounts for the door speakers and faceplaces for the HU. I just cut my own using the existing as a template, but would definitely use his if it had been available when I put mine in.
http://forums.corvetteforum.com/show...nc&forum_id=20
And the audio section here is pretty good at answering or giving advice for sure.
As the other guys said, if I can answer anything else or anything let me know.
TJ