Lots of people have said that the next GTA game will be called "San Andreas", because the first GTA game had three cities:
Liberty City
Vice City
San Andreas
So, I think the rumor is pretty well founded... But I have no idea whatsoever what the theme of the game would be like. I think it would be pretty cool if it took place in the 60's or something, but I think it would be super super cool if they made a sci-fi GTA game, with hovercars and stuff.
But about being slow, don't worry about it.
I beat vice city a few months back, but I only beat GTA3 for the first time a couple days ago. I found GTA3 to be WAY harder than vice city, and longer also. But overall, I liked Vice City more.