Its an growing subject these days with people overclocking their processors (and having to add various cooing systems) to people slotting their computer into fancy boxes (check out the resources below)
Overclockers.com - if you want information on running your processor at top speed and the colling systems that are also required (including water cooling) then these the people you need to visit.
AppleFritter - For the Apple / Macintosh crowd looking for case mod ideas.