- If this is a pure code breaking application, you are probably better off with FPGAs than GPUs, but it is very easy to gang together a few Xboxes. FPGAs are harder to come by.
Looking at the price of the FPGA with the design of the custom board, why not going for a (or more) Nvidia Tesla board C2050/C2070? (price is around 3000,- USD per board for 448 GPU core)