I would not go with the Hyper N520, as its technology is outdated. You want to go with a HDC style heatsink (Heatpipe Direct Contact), especially with the brand CoolerMaster.
You will have to remove your motherboard and components anyway to install either of the heatsinks mentioned, so why not upgrade your case while your at it? Its a shame to have to compromise performance because of a few millimeters.
CoolerMaster HAF 912 is on sale @ newegg.com for $59 ($49 after rebate).
And I would recommend the CoolerMaster Hyper 212 Evo, you can find @ the same place for $35. (not to be confused with the Hyper 212 Plus, big difference!)
I may be slightly biased because I own both, however I think I deserve to be with CPU idle temps @ 20 Celcius & full load temps @ 35 Celcius.
You could go liquid cooling, and probably keep your current case... but for the money, I wouldn't.