No need for water cooling, the Hyper 212+ is fine, its a great cooler for a low price of 25$ on amazon or 30$ on Newegg. Also, you did clean off the compound correctly right? You have to use 75% Isopropyl alcohol (spelled something like that) or anything like 100% would be fine too. After that I'd recommend using AS5 or something better like MX-2. I personally use IC 24 Carat. Anyway, anything above 80, is way too high, even above 76 is running hot already. You want your load temps never to pass 75 like J-Sparrow said.
I hear many people get to 4.0ghz with Turbo. But If you running 24/7 or at least you don't want a spike in your bill, don't oc to 4.0ghz The power draw is a lot more than 3.6ghz something like 435watts-ish. So 3.6-3.8 is a good OC.
If you find the Hyper 212+ not cooling enough, get 2 Scythe Slipstream 1200rpm fans and dual fan configure it. There "quiet" (not the quietest obviously) and way quieter than the stock fans on the Hyper 212+.
IF you want a recommendation for a higher quality cooler, the Venomous X or X-RT is good, the X-RT just includes AM3 brackets and a Thermalright fan, but the overall cooling performance is fantastic. However I would think that the N520 you have is enough for a good OC.