I could only add 40/50Mhz to my 660, but it was already overclocked as it was the EVGA Superclocked. I can't remember what the speed was out of the box though, possibly 1150Mhz. You won't get too much extra performance but it's nice to have a free boost if you can get it stable. Try adding 10Mhz at a time and test if it is stable.