Idea to avoid power issues - some users have reported that certain PC 7800GT cards (notably from eVGA and PNY) work in the mac pro to drive secondary displays.
Since these cards take external ATX power, it may not be necessary to try and push extra juice over the PCIe bus.
Just a thought.
I agree, I know we're going to need external power somehow. External power supplies is the way to go, even if it means an ATX supply