I beat III last night. If you don't get all of the secret packages (which I didn't,) and you don't know the cheats (which I couldn't get to work,) you kind of have to plow through the final mission until you figure out which order to kill the bad guys. The most important thing for me was to decide which 5 guys to snipe, seeing as how you only get 5 shots.
It is strangely unfullfilling to waste so much time on a game, finally finish it, and then be left thinking, "well, that's it. It is over."
I've got Vice City still sitting in the box, so when I get the xbox fever again, I'm sure I'll try and do to Miami what I did to New York.