The silentx stuff is mostly wishful thinking.
http://forums.silentpcreview.com/viewtopic.php?t=16393
spcr is slowly testing fans.. the nexus 120mm...
12V : 38~41 cfm : 22~23 dBA/1m
9V : 31~33 cfm : 19~20 dBA/1m
7V : 23~24cfm : <18 dBA/1m
5V : 15 cfm : <17 dBA/1m
nexus $20 the globe is $8... i woudln't bother trying anything else, at least other dc fans.