Your HSF is plenty capable of cooling an overclocked CPU. Just make sure to be very conservative about raising the voltage levels and you'll be fine. Your motherboard has an easy overclocking feature called CPU Level Up, but as many have pointed out ASUS is quite generous with the voltage to make sure those clocks can be reached. I would see what it's changing the settings to and use that as a starting point and then start lowering voltage in small increments until it's unstable and then bump it back up to the minimum voltage where it is stable. Get Prime95 to stress test your system for stability. You should be able to get 4.4-4.6 GHz without much trouble. Don't try to force it to do 5GHz, that's where you'll start running into problems that can cause damage to your CPU if you aren't careful. Very few people have an Ivy Bridge chip that can safely do 5GHz or more on air cooling.
Intel Core i7-875K, Asus P7P55D-E Pro, Win 7 Home Premium
MSI GTX 560 Ti OC, Mushkin 2x2GB DDR3-1333, Corsair TX650
Cooler Master Hyper 212+, Logitech Z-2300, ASUS Xonar DX
Samsung Spinpoint F3 1TB, Dell Ultrasharp U2410, Antec P183