I would be wary about using a molex to 8 pin PCI-e adapter for such expensive equipment. However this might be better than the dual PSU set up I mentioned.
We would really need to see the power draws of the 7990 to tell whether or not those adapters would be ok (and which PSU it would go well with).
Thanks, Chainzsaw! I'll be using the machine for photorealistic renderings of the effects of different lighting technologies on architectural structures. I was looking at using the dual Xeon EVGA SR3 Super Record, which says it supports 4 way SLI (assuming CrossfireX, too). The Strider ST-1500 only has 4/4 of 8/6, so probably a second ST-1500 will be needed. I think I found a cable that will allow me to sync them.
Thanks, again, for pointing me in the right direction.