Do you have an owners manual or is there a manual that came with the car concerning just the stereo? If so, either one should let ya know if there's an amp.
As far as changing out the speakers...That's up to you and what kind of sound you're looking for, NTM if you want to remain as close to original (stock) or not.
My wife has a 2000 c5 and her Bose system doesn't
sound as good as the one in my 2005 c6 IMO.
So if you have the lower level bose system...upgrade it to the better one. Heck, you can probably find a used one for sale on one of the forums.
JAT...If ya buy a bose system (New) you'll probably shell out more $ than if you were to just replace it with another known brand system.