ExtremeMHz have been busy building something of a leviathan, in their project to get a dual motherboard, water cooled ass-kicker on the go, which you can read about here.
Its worth reading even for their choice of case, which is about the biggest PC case I have ever seen. Heavier metal than that tends to move into the realms of rack mount cabinets (which could contain servers, switches, routers) or small mainframes, etc. The final machine has got some pretty good cooling, and after adding all kinds of things I had never even thought of, they send up with something of a monster.
A well worth read here.