In Mark's famous folding farm thread he was using bare motherboards sitting on a rack in the computer room at his work place. They were basically a mother board, CPU/Fan, RAM, and a power supply. In fact, he was splicing in an extra power harness and running two mother boards off of one power supply. Nothing else, not even a case. He was running them on Linux, diskless, headless, etc and booting them over the network. Towards the end, his avatar was a pic of the computer rack.
EDIT: I can't believe it, I actually found a thread with pictures of Mark's Mega Monster folding farm
, as he called it. In the last post of the thread, Mark said he was using ECS K7S5A mother boards with AMD Athlon XP 2100+ CPUs. I was wrong about Mark using one power supply for two mother boards. While researching this, I found out it was dwjj who used to do this. Here is the SS51G cluster
referred to in the second post of Mark's Mega Monster folding farm thread. I wonder how many points a day that cluster could crank out.