How much RF-interference does a typical desktop computer produce? How much does a metal case without windows reduce it?
Well I don't really know.  But two ideas to measure (or compare) it.  First if you have a 'scope then clip a bit of bus wire on the end of your x10 scope probe and move it around... take off the case, put it near the over head fluorescent lights.  If you don't have a 'scope then you can do some with an AM radio.  Tune to a blank spot and try and hear the noise as you move it around.  (The radio idea comes from here.)