That build is definitely overclockable. The Hyper 212 Evo can handle a mild overclock (like 4.0 - 4.1gHz) without approaching high temps. If you want to go higher, a bettere air cooler is necessary. If you want the most out of it, a custom loop is required. Don't think that you need a custom loop, though. I recommend getting a Phanteks PH-TC14PE or a Swiftech H220 when it comes out. The H220 is very good - practically a custom loop that is just pre-assembled.
yeah that would be ok for mild overclocking, im not sure youd want to go much more than that with a 520Watt PSU, under heavy loads for the cpu and gpu it "might" start to struggle.