I have 15 of these delta 120s - 252CFM - lovely for shifting air BUT too loud for continuos use (unless its in a server room) i have 12 pulling air in my server cab and 3 to 'play' with 
I have a couple of those and they are just insanely powerful: http://www.youtube.com/watch?v=kDOSaPULFro
that noise ....
just one in my office is too much!