These are the spec for the graphic card's power supply.
The Mac Pro only has two 6pin power source, each of them are rated up to 75W.
A single 6pin power plug on the card may draw up to 75W, and a 8 pin up to 150W.
Therefore, theoretically, the cMP can only power a single 8pin card, or 2x6pin card.
Some 7950 are 2x6pin, that will fit the cMP.