Personally i'd go with a Cooler Master Hyper 212 because it's a great air cooler for the price and should serve you well with moderate overclocks. If you're looking to spend a bit extra, the Noctua NH-D14 is about the best air cooler out there ATM
I am getting a gaming/entertainment pc and I need a cpu fan because I am overclocking it so is the "Xigmatek Dark Knight Nighthawk" a good one? Thanks.
I read one review of that Dark Knight II CPU cooler and ever since then, all of my builds that I build on pcpartpicker.com, include this very cooler. It's price is only 39.99 and the review says that it performs close to a 100$ water cooler. It's from a great name brand, and the cooler sure does look great. I say go for it.