AMD 6600k + cooler master hyper 212 EVO?

overclocking is always a gamble really. and most of the time, the cooler itself (as in, any big tower cooler) won't make a lot of difference. it's much more important to get a good motherboard and apply thermal paste properly:
http://www.tomshardware.com/reviews/thermal-paste-heat-sink-heat-spreader,3600.html
http://www.tomshardware.com/reviews/thermal-paste-performance-benchmark,3616.html

with that said, you can probably get to 4.5ghz or slightly above. especially if you don't use the integrated graphics making room for the extra tdp