Hello .. I see you have not been answered , I am also very new to over clocking gpu's but I to have gtx680m ,
So I use msi afterbruner , and the setting I found good and stable was , core clock + 135 and memory +650,
I was advised to start at +400 ( memory ) and keep increasing and testing in 50mhz Lumps , an till it crashes , then at that point move back 100mhz ,, and now you have your setting mine came out as show above .
Thx to Meaker for this info , I am simply reiterating what Iv been shown ,, I hope this helps , it helped me loads .