why do you think that multiple 5970 fry motherboards?

[undeath]

I'm not sure why you would want to specifically use 5970s in the first place. I'm not even sure AMD still supports those (and consequently hashcat). Granted, even newer high-end AMD cards have this problem.

If you only run two cards and are not running 24/7 you are relatively unlikely to fry your board. However, I recommend buying recent Nvidia cards over those broken-ass AMD ones if you want to do hashcracking.