I am running the i5-3570k at 4.5GHz on the same cooler, with two extra fans I put on it.
If you want a slight OC, 4.0GHz is easy enough, and won't require much extra voltage.
This guide is what I used to overclock, and this is my first time overclocking:http://www.overclock.net/t/1198504/complete-overclocking-guide-sandy-bridge-ivy-bridge-asrock-edition
I'd also suggest (AFTER finishing the guide and finding a stable overclock) setting a lower offset for the vcore (negative number) and then increasing the turbo boost to keep the max voltage the same. This will keep your idle voltage low and your load voltage where it needs to be for the overclock. Keeping a lower idle voltage will keep from straining your CPU too much.
I also noticed my CPU fan speed controller defaulted to full-on. Might be worth dropping it to auto so it isn't like having a jet in your house 24/7. I was able to maintain around 80c with fans on auto.
And lastly, here are some SC2 benchmarks I did, along with temperatures:http://www.tomshardware.com/forum/281771-29-overclocking-3570k-starcraft-benchmarks