I have 24 U rack waiting to house 4 nodes, each 4 double graphic cards (8 GPU-s), total 32 GPU-s. Combined arithmetic performance is about 28 000 GFLOPS = 28 TFLOPS.
I have 4 shelves (4U height to house 1 or 2 PSU-s each, system board, 4 GTX 295 graphics cards and hard drive. Trickiest part was to mount system board to shelf. Holes didn’t match. I bought 3 mm bolts, and 20 mm spacers to make good mounting. HDD is seated into plastic HDD shelf, attached to shelf.