7V is widely regarded as a good compromise between speed and noise in a PC situation - I believe you can get it by hooking negative up to 5v and positive up to 12v - the difference gives 7v across the fan.

Don't do this without testing it on a powersupply first though, or waiting for someone more knowledgeable than me to state whether this does work, or will end up blowing something vital

Cheers,

A.