Well, firstly, a core 2 duo is better than a pentium 4. Forget about the 'GHz' number. You can't cross compare different processors, because under the lid, they all work differently.
It's like trying to say an 18-wheeler is faster than a racing motorbike because it has more horsepower. There are other points to take into consideration other than pure power.
Secondly, how much of an improvement you see over single core depends on the game itself. For a game to take advantage of a multi-core processor, it has to be coded from the ground up to have various parts of the code processed on different cores, something which is apparently very hard to do. Obviously, some games will be coded much better than others