I've been thinking of building a new PC desk (it's in the same room as the home theater/2 channel seating).
The main "hush-box" that contains the PC and the external USB harddrives would be lined internally with Owens Corning "703" (acoustic wall treatment material).
Also, a rubber isolated floor inside the box to minimize physical/mechanical vibrations to the rest of the desk.
Two holes;
One to allow air in with a super quite fan.
One to allow air out. Possibly directed towards the carpeted floor to further minimize noise.
........For what it's worth.
Bob