I agree on the 120mm fans. I was used to my old Athlon system that mostly used 80mm's, and the 12cm fans are now almost inaudible.
I use several of the Japanese Scythe Slipstream 500 upm fans, they dont move that much air but more than enough. 500 upm are practically noise free except some weak air stream, and they don't cost much (~8€ per fan).
http://www.silentpcreview.com/article832-page1.html
But I'm not interested in water cooling, they still use fans, and I can get a cool and quiet system on air for much less money and hassle.