This will make your quiet computer effectively silent
Isobox - expensive, with thermo controlled fans etc. 30+ db reductionhttp://www.custom-consoles.com/isobox.php
They're made for music studios.
Short of having everything passive, or putting it in another room, I don't think anything else can be as effective
You sure can't passive out a DVD drive
And you can make your PC live a bit more healthy with the fans running faster