Please use an aftermarket HSF. Otherwise, forget about it.
Raise Vcore to 1.30V
Enable Internal PLL overvoltage
Disable VDroop
Raise CPU Multi to 40.
Use RealTemp to verify CPU temps.
Use Linx to load the CPU, using max. memory setting, 5 passes for starters.
If successful, raise the Multi to 41.
Raise until unstability in Linx (or BSOD), then raise the Vcore.
Retest. If successful, raise the multi again.
My 2500k is running at 5.0GHz with 1.375v Vcore, using a Thermaright TRUE Cu.
You can also check this guide, well written:
P67 Sandy Bridge Overclocking Guide For Beginners