As far as your design goes for stacking. The middle card is not going to get any airflow as it is currently positioned. Might I suggest stacking the boards so the heatsinks are sandwiched together facing each other, with two cards per fan. It would also be a good idea to use 120mm fans. More efficient, less noisy than 80mm.

Also, while I appreciate what you are trying to do, Who are you? No post count, late '12 join date, and no reputation make me extremely apprehensive sending you 100's of BTC worth of hardware.