if they made desktop motherboards with 8 pcie slots than definitely 8xhd7970 than say 4xhd7990. not to mention you'd have to build a custom case using pcie raiser cables, multiple power supply, etc.. can get complicated. the biggest advantage with four dual gpu cards is that you can fit them in a standard full size ATX case using a single high wattage PSU without any customization. the trade off is it won't be as fast.
on a related note, i've been reading of bitcoin miners making the switch to FPGA from GPUs. mainly due to significant cost savings on power since they run their boxes 24/7. if you plan on a lot of hash cracking and will be running your computer 24/7, then perhaps it might be an option for you. of course you'd have to look for another app that support FPGA and the hash algorithm you want to crack.
on a related note, i've been reading of bitcoin miners making the switch to FPGA from GPUs. mainly due to significant cost savings on power since they run their boxes 24/7. if you plan on a lot of hash cracking and will be running your computer 24/7, then perhaps it might be an option for you. of course you'd have to look for another app that support FPGA and the hash algorithm you want to crack.