Depends , if it's a 9xx series you could take it to about 3.7 , if it's a newer Sandy Bridge take it to about 4.2
Just make sure you know what you are doing , so you don't **** it up.
wut?
i have my 950 and i got it to 4.2ghz.
i cant take it further yes,but with usafe voltages.(its watercooled though)
just be use to enable load-line calibration, it kills the vdroop and adds stability.
ps: you aint gonna overclock with the stock cooler are you?