I mine with an exactly similar box, and I just enable crossfire and don't need a dummy plug. Works fine.
i think it depends on your OS. windows or linux?
I for one intend to use Windows 7 Ultimate 64 bit.
I found another guide to building a dummy plug:
http://soerennielsen.dk/mod/VGAdummy/index_en.phpThe thing with crossfire is that I read on the forum that it decreases performance a little bit. I think for a dedicated miner squeezing as much out of the hardware is very important. But TheKid if you can assure me that performance won't suffer *that* much, or even give me some numbers to consider (ie with crossfire and without), then I'll definitely not bother building a dummy plug.