The 670 is almost as fast as the 680 to begin with and that's with no overclock. What would be the purpose of to outbeat the 680? You won't notice any difference with the additional fpsIf your playing BF 3 and with the 680 your getting 60 fps and you then overclock the 670 in the same game with the same settings and get 65 fps you won't be able to tell the difference with the naked eye and you need a software program to tell you that your getting more fps.
If you qare determined to overclock the 670 then it won't take much to go past the 680.
The first link shows the cards to be tested and the core clock speed , the Evga SC has a core clock of 967 mhz and the reference clock of 915 so the SC edition comes factory overclocked.
The next link shows the benchmarks of the cards and you will notice how close the SC is to the 680 with both cards using the same settings in BF 3. With the results of this benchmark you can assume that an overclock of the core clock running at 1006 mhz will push the 670 past the stock 680.