personally i have a Noctua NH-C12P and i love it... it cools my 8Gbs of ddr3 and set to very low i have temps of 45 idle and 55-60 maxed out... if you have a horizontal case, the heat pipes will provide even better cooling
However it's orientated so the air flow goes to the top of the case which was no problem for me since my case (COOLER MASTER HAF 922 RC-922M-KKN1-GP Black Steel + Plastic and Mesh Bezel ATX Mid Tower Computer Case) had the largest fan on the top