Im curious about the same thing to setup 2 4870's in crossfire on the mac pro 2008 model. I was thinking of using a 5.25" psu with 2 pci-e 6 pin to power it. FPS provide a 300w model which would be perfect. There are 2 16x PCI-E slots available so no bottleneck. It would work theoretically.
Two work with either external power or power split from the optical drive bay.
Oh, but you just don't have the cards yet, correct?