Umm depends on what fans you want search new egg for fans and they will give you dba readings. Anything between 15-20 is extemly quite for a fan. And 20-25 is also very quiet.
eh.
i dunno. i have some vantec stealth fans and they're supposed to be silent. I bought 4 of them and i can still kinda hear them. but only if i'm listening for it.