I'll be interested to hear your impressions of this rig once you get it up and running. I had a Crossfire rig back in the day based on the Asus A8R-MVP (Radeon Xpress 1600 NB/ULi SB) and two X1800XT's (a master/slave card setup), and it was the absolute bar none worst PC I ever built.
CF was in its infancy, and 'vpu recover' was a (very) common occurrence. To make matters worse NVIDIA had just bought ULI, so driver support for an already spotty south bridge, was almost non-existent.
The whole thing soured me on Crossfire for while, and I didn't even consider it again until I picked up a 3870X2 two years later.
Anyway, be interested to hear how it works out for you.