I was searching the net for a solution to the limited GPU space for one of my systems, and came across a reference concerning high current draw problems when running 3 or 4 ATI/AMD 5xxx. 6xxx and 7xxx series cards. This probably also applies to high-end CUDA cards.
Read about the problem and its solution on Zorinaq's blog (here)
Also found a cheap source for PCIe x16 Bus extension cables on eBay (here)