You might want to do some research at www.silentpcreview.com.
Here is a link ot their cpu cooler recommendations: http://www.silentpcreview.com/Recommended_Heatsinks
I recently installed a Noctua NH-D14 and am very pleased(but it is huge).
On low speed, it is all but inaudible.
Once you get on the quiet computing kick, there is no end to it.
You will next be looking at gold rated psu's with fans that hardly run.