Doubt much would change. If anything, Giuliani might help Bush in swing-states that went for Kerry and help solidify swing states that went for Bush. The map, other than some margin changes, might look pretty much the same.
Same, Giuliani would slightly be a better campaigner than the flat Cheney. I could see maybe that little extra boost across the nation could flip New Hampshire, but that's about it. Ohio would still in some way be contested by some, not by others and in the end Bush would probably be reelected.