can i use an external power supply to power second graphics card?

hello. i currently have an ATI 5870 in my 2010 Mac Pro. I want to throw a Quadro 4000 in there as well, but i need one more 6-pin power cable. can i plop the quadro into the second PCI slot, buy a PC desktop power supply and just let it sit on the outside next to my Mac Pro, and then just run a 6-pin power cable from the power supply into the Quadro? i realize that the Mac Pro door would could not be placed back on...but i could figure out some way to cover up the opening so that dust doesn't enter.

would this work? anyone tried?