People please read on Nvidia 3.0 GPU boost before recommending pascal graphic cards. How much you can manually overclock your GPU DOES NOT MATTER anymore, at least not for Pascal because BIOS hacking tools haven't been written yet for Pascal. The guy who wrote Keplar and Maxwell tools is clearly having trouble backwards engineering everything, cause it's taking him a long time this generation and might never be realeased.
You can't manually overclock more than 3.0 GPU boost technology can give you which is the same clock for all graphic cards atm. GPU to look for should be the coolest one (lowest temperatures), not the one that has highest manual overclock.