GTX570 is already powerful, if I were you I'll just leave it as is no OC, besides you'll just shorten it's life. Just run BF3 on auto. Of course it's up to you if you wanna OC that, just a friendly advice cause that is an expensive card.
How's gtx 680 working? Been watching reviews on it and it performs like a magnificent beast. I would even buy just that card and display it without using it