Have a look at the SPCR 120mm fan round ups and there stuff on the Anatomy of the Silent Fan
120mm Fan round ups
http://www.silentpcreview.com/article695-page1.html
http://www.silentpcreview.com/article695-page1.html
http://www.silentpcreview.com/article832-page1.html
They really like the Scythe Slip Stream fans and got better results out of them than the nocturn NF-S12
With the 800rpm version pushing out slightly more air for slightly less noise than the nocturn
Although they've not reviewed the other nocturn models and all told you'd not go wrong with ether.