I am planning on buying the 670 FTW Sig 2 because of it's great cooling technique and its good core clock (1006 Mhz). If I want to overclock it, what is the max I could overclock it to keep it running on good temps?
The cooler is really the only thing. You can't bet on any brand overclocking better than another. I wouldn't look at the core clock either, I'm pretty sure even reference boards are binned well enough to attain 1050Mhz, would be dangerous to put out cards that were unstable that close to the 915Mhz base clock.
That said, if you are looking to overclock a 7970 might be a better pick. Its a little faster than a 670 at stock, the same price, and guaranteed to overclock far better because of the unlocked voltage.