One of the best guides I have found
is this one here.
According to the post the key is keeping your 670 under 70c to prevent throttling, and to understand how the boost works.
The post describes using Precision-X as the tool to overclock and that's what I used, (before that Afterburner). Be careful of the EVGA K-Boost function, it has it's good and bad sides. The good is that it keeps the maximum voltage (1.175v) on at all times, but this is useless when your not gaming; it just increases temps and fans. For example both gpus are sat at 71c and 64c AT IDLE with fans doing what they want on auto (44% and 38% respectively) with Precision-x closed. Not a good pratice. To disable the K-Boost you have to disable SLI then reboot the system. No good if you are jumping out of game, browse or do something productive, then game again. It wouldn't be so much of an issue if on water, apart from the added power consumption and potential wear on the gpu.
My two EVGA 670 SC 4gb models have been great performers but not so at getting any further gains on the boost - they just get too hot. When overclocking with fans at the max 80% and all case fans at 100% (in a FT02 no less) the cards still reach 75c during Unigine Valley. The noise is stupid so I'm going water cooling next month, but that's another subject.
I haven't tried the cards individually, but together I can get a gpu offset of 52 but nothing on the memory to be stable, or a gpu offset of 30 and a memory offset of 400 stable! I stopped trying to get any higher than that on the memory as I'll revisit the overclocking when I do eventually get them under water.