A cluster with 100 nodes, each with 2 dual-core Opteron 285 @2.6Ghz.
I was getting between 400 and 500 MH/s, couldn't occupy it for long and exclusively.
But a Radeon 5870 GPU in my home PC does 350 MH/s.

Compare:
Home PC - a small quiet box for ~$1000, 100 nodes cluster - a room with 8 noisy stands for about ~$1000000.

GPU mining is 100-1000 times cheaper for the same performance, both in money and in energy.