Im running mine at 4.8ghz, 1.46v at about 68c with a thermaltake water 2.0 extreme. From what I've read you dont want to exceed 90c, even tho I would never let mine get that hot.
I have a I5 3570k & Finally got it Overclocked to 3.2ghz to 4.2ghz on Stock Voltage!!! I wouldnt Use Stock Heatsink to Overclock your CPU its Bad for it I Recommend getting a Hyper Evo 212 or Water Cooling to keep your Temps down you can OC more!!