I mine with an exactly similar box, and I just enable crossfire and don't need a dummy plug. Works fine.
Yes, but I have a 5850 and a 5770 so I couldn't do crossfire and I have to use the dummy plug.
Riiight, now I get it. OK, well in that case I think I will stick to just linking the cards via Crossfire. I see no point in complicating things if I do not have to.