Gore would have won, I think. Graham would have won Florida for him, and I don't see any states that Gore won with Lieberman that he would have lost with Graham.
The choice of Lieberman was an attempt to distance himself from the Lewinsky scandal, since Lieberman was the foremost Democratic critic of Clinton's actions in the Senate. I don't think Gore was really helped much in that area, however.
That was a dumb move, as anyone who would care about a BJ in the White House is going to vote Republican anyway.