Ah, I couldnt tell from ati's site, so your card requires a power plug just like a hard drive or cd player right? If you can, use one that isn't plugged into anything else. If you can't, I'd suggest using the one that feeds a cd rom or something else that isn't likely to be running all the time, and try to stay away from any that are plugged into cooling fans. I don't THINK that would cause a problem, but you never know.

I'd guess your MB is having an issue working with the card because of voltage, or because the MB doesn't handle agp8 correctly yet. Did you check to see if there is a beta bios fix out for your MB that is supposed to patch agp8 functionality? My ASUS did, as well as my biostar.

Best of luck, try that jumper on the MB, I'm guessing things will work a little better after that.

/em crosses fingers.