without outlaying a pretty decent amount of cash, its probably easiest to do some type of water cooling on your computers, and as bigluv pointed out, minimize heatout put. if you do a lot of video processing or the likes, that wont help much.
but to answer your question with a finite answer; these will do exactly what you are looking for:
http://www.blackbox.com/Store/Result...s/n-4294954894 <full size
http://www.blackbox.com/Store/Result...d/n-4294954891 <small sized.
doing it in a closet would probably require some kind of permanent changes to your house and would probably end up costing about the same without the ability to take it with you when you leave.
been there, learned about that.