Could you not do Crossfire under Windows, assuming power is not a problem? I've seen some "small" PSUs that fit in the optical drive bay, I'm sure one could power one of the 4870s and the regular power supply the other.
I would not do it because it blocks out one of the PCIe slots, which means that I can't plug in all of my necessary components that I would BUY a MacPro for...
Or am I rambling?