A lot depends on quality and efficiency of the PSU. My S3's arrived today, I only had generic ATX 550W PSUs handy (pci-e wires for server PSUs didn't arrive on time), I hooked it up to generics and a single S3 unit wouldn't power off a 550W PSU. I had to split the load between two 550W PSUs. I then plugged one 550W watt PSU powering only single blade of A3 (one with WRT/config card) into socket with load and voltage reader and a single blade running off some generic chinese power supply is drawing 249W, so far cry from manufacturers figure of 360W per unit, 180W per blade. Obviously losses on 12V rail in this particular PSU must be epic, but fair bet that 750W version of PSU with such terrible design wouldn't be able to power two S3's, you could barely power up one.