You don't really need to OC them. Two gtx670s are powerful enough for most recent games, unless you are planning to go multi-monitors.
My first 670 is Asus DCII top (not OC) but it died after a couple days and I replaced it with two galaxy 670 GCs. I heard that some people have the same problem too. Currently where I live there is not a single store with Asus DCII top cards in stock. I believe there must be some problems with this model, but that's just me. If you want to stay on the safe side I'd say don't OC them.
BTW galaxy 670gc is surprisingly as powerful as Asus DCII top. So I guess the extra money I paid for Asus went to the backplate and not performance increase.