This will make your quiet computer effectively silent



cheaper
http://www.silencecases.com/pro_attenutation_cases.html

Isobox - expensive, with thermo controlled fans etc. 30+ db reduction
http://www.custom-consoles.com/isobox.php

very expensive
http://www.norenproducts.com/products_acl.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