as a sound-proof case. To be truly sound-proof the case would also have to be air-tight, and that isn't a viable proposition. With a case that has to be cooled by exhaust fans, air needs to move in and out of the case, and therefore sound will do the same.
There are lots of things you can do to make things quieter however, and one of them (silent PSU) is outlined by goonerbill.
You can alter the harmonics of the case very simply - just stick strips of thick adhesive tape to the inside surfaces. This works on the same principle as the patches of black material that are stuck to the underside of metal sinks - they are there as dampers to alter the harmonics of the metal structure so it doesn't make such a clashing 'metallic' sound when hard objects come into contact with it.
You can also substitute 'quiet' fans for the standard ones on your CPU and graphics card, and you can alter the position of the case. By this I mean move it to a position where sound is minimised. You would be surprised at the difference this can make, particularly if the case is in a corner of the room - that's the worst position from the noise point of view.