I have the 580 working great on windows only... Heres what I did. I used the two mini 6 PCIe and the wire they gave you to hook up to the 8 port. as for the 6 port PCIe on the video card. they give you a cord that goes from the 6 port to 2 of the older non-mac power cords... I bought this on amazon and hooked it up to one of my unused hard drive connectors.
http://www.amazon.com/gp/product/B002LBROAA
you do have to but off part of it just so it plugs in, but dont worry it hooks up perfect!
Oh, you may need two of these if you don't have them already:
http://www.amazon.com/PCIe-Power-Cable-nVidia-Video/dp/B002UR1654
they hook up to the mini 6 PCIe on the motherboard just below the PCI express slots.
Hope that makes sense!