hey, im new here and dont know a ton, but I just set up a rig with 2x 5830's and they are running fine, but no crossfire. have you tried removing the crossfire cable? I dont think you need it for a mining only rig. I dont.
Quite correct, crossfire is not good for mining.
Yup, google VGA dummy plug. That is what you want, and you can make one using $1 of parts from Radio shack.
You don't even need one of those if you are running Linux.