Lightening02 :
The core i7-6700 is a processor, not a graphics card. You can't overclock the non-k 6700. However, if you had a motherboard with a z170 chipset and a core i7-6700k, you could overclock. In that case 3.4 GHz would probably be pretty safe.
you can overclock i7 6700 (like all non-k skylake) on z170 and even with some b150/h170 from asrock, the hyper series
with asrock z170 motherboard u can oc even with latest bios, on other brands u must get an specific bios from web
i got my i7 6700 at 4,6Ghz with 1.35v or even 4,8Ghz at 1.45v, but too much voltage for my opinion on daily basis
my brother got an i3 6320 at 4,8Ghz with 1,32v .... lucky cpu