PCIe Slots and power?

Hello all.

I would like to install 2x 7950 cards to do a BruceX test..... but I am a bit concerned about the power? I am waiting for 2x power cable splitters.
www.amazon.com/dp/B004NNTVT6/ref=pe_3850...309540_26725410_item

concerned this will draw power from the motherboard and maybe fry it???but I should probably get a dedicated power supply?? any suggestions appreciated.

From what I have read, slot 1 and 2 are more powerful at 12lanes and 3+4 are at 4 lanes?
...then I found this:
support.apple.com/kb/HT2838?viewlocale=en_US&locale=en_US

which made me more concerned about this test...

I notice in my istat menu results (GFX-cards.jpg)

indicate that all the slots are 12v-lanes? and 1 and 2 are boosted?
(screenshot while rendering realflow and c4d)

Spec are in the screenshots.

cheers in advance.