Ok I hope this will help. The side by side is if you scroll all the way to the bottom. But read eveything in between. From what I have read none of which said anything about GDDR3 for both the cards.
But anyway it should be close to the same results. I get two mixed answers on this. In some sites they say the 6750 is only 36% faster and in other sites it tells that they are even in there abilities.
So I will link you to this site and it should shed some lite on it. Just remember that the 6750 is a rebadged of the 5750. But if your case is a mini then you will have to find a lowprofile card.
I know they make them in the 6670 but I don't know about the 6750. Anyway I hope this helps and good luck to you.