How aggressive do you want to be in overclocking?
What is your budget?
Your case can can handle a cooler of 161mm.
A great $35 cooler is the scythe kotetsu
Here is a review:
http://www.silentpcreview.com/Scythe_Kotetsu
You really need no more than a noctua NH-U12s.
For the most aggressive cooling, look at the noctua NH-D15s