The case for FPGA boards as I understand it:
ATI 5850 can give about 400 MHashes and uses about 200 watts. (ie at best 2MHashes/watt)
Dual FPGA X6500 gives about 200MHashes and uses 20 watts. (ie. at best 10MHashes/watt)
If we were to keep 2 rigs, and sell the other 11 at... $450? each.
Why would you keep any rigs? If 1 rig is profitable then all the rigs are profitable. If 13 rigs are unprofitable then 1 rig is unprofitable.