But they are stupidly overpriced. And the Beige version is 3U, because you just have to fit the original PSU into it
the thing about the beiges is they had a 230W ATX PSU which ran the mac and a crt. Run them headless, and I reckon a 500W ATX PSU would power 4 or more of them with a simple ATX splitter harness (not too hard to rig).
I just made some cases to rack mount two beiges, but they are more like 2U or maybe 2.5U to fit the height of the RAM and CPU with heatsink.
I'll let you know how they turn out when I find my dremel to cut holes for the ports and to mount the components.