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The dual 6 pin pulls 75 watts from each header so you get a single 150w 8 pin output. The PCI slot itself also provides 75w I think.
The card should actually work just fine with the 6 pin cable from the stock 5770 as long as you plug it in right. I have the 4GB Armor edition of the same card and I ran it with the stock cable for a few days while I waited for the dual one to show up. Worked fine, but I used the dual cable just to be safe.

Unless you want the gaming specs, you can always get the cheaper 4gb 560 model Apple recommended. It doesn't even need extra power, it gets what it needs from the PCI bus.
 
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