It depends on the individual chip/card, milage varies as they say, one card will go from 800MHz/900MHz stock to 1GHz/1400MHz, the very next one off the line won't move from stock settings without problems,
there is no fast clock, take your time and see how far you can push yours comfortably, http://www.guru3d.com/article/geforce-gtx-680-overclock-guide/1 may be of use to read through
Moto
It depends on the individual chip/card, milage varies as they say, one card will go from 800MHz/900MHz stock to 1GHz/1400MHz, the very next one off the line won't move from stock settings without problems,
there is no fast clock, take your time and see how far you can push yours comfortably, http://www.guru3d.com/article/geforce-gtx-680-overclock-guide/1 may be of use to read through
Moto