With every gpu chip being different your not going to get a sure fire answer, your just going to have to do your overclock the way everyone else does and that's one step at a time untill you reach the limit that your chip has. There are microscopic differences in every chip that make them unique and while there may be people that get the same mhz in the end there are just as many that will get a different mhz so just do your overclock and see what you get.
Try this tutorial, it goes over EVERYTHING needed for GTX 670 overclcoking, and takes into account that each chip is different. I followed this and god great results in real-world BF3 games, and synthetic benchmarks like Unigine Heaven. Sorry I don't gave benchmark results as I deleted all my benchmark tests since doing this, but it is a wonderful overclocking guide. http://www.overclock.net/t/1265110/the-gtx-670-overcloc... (BTW, I used MSI Afterburner to enable my card's triple overvoltage feature)