Just a note you could always build them in sets of 3 and have them mine to one machine(If pool mining put your credentials in all of them, if solo mining have a host computer with your coins dameon on it and set it to mine to your ip address) May be more expensive but as for heat, effectiveness(7xcards are a lot more likely to crash than 3) and difficulty it would be optimal

first i will not put another card in, its just a example of a maximum you can get, over 4 card in a box its hard thing to do