Notester82: I already own Rollercoaster Tycoon on a CD for my computer but I don't have any expansions. Once I saw that I could buy RCT with expansions on GOG.com, I decided to think about buying it. Will my saves from RCT still be there if I buy RCT w/ expansions and put it on my computer?
They will be there unless you remove them, and they will work in the new copy if you didn't use any hacks and advanced trainers.
Check where your current rct is installed, there should be a subfolder "saved games", copy that one to a safe place. I'd uninstall the old game before installing the gog version, hence the copying; if you decide to try if they play well both installed that isn't even needed.
The /tracks folder probably also contains stuff you want to move over.