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Yes you can. You'll need some molex cables to run power from the second optical drive bay to the card, however.

I'm doing it in my 2006 Mac Pro with a 5870 and an 4870.
 
If it were me, I'd play it safe with a PSU mod in the unused optical bay. Apple doesn't publish power rail specs for their PSUs, so to me, it's a shot in the dark over the long term. I would be worried about over-stressing the Molex feed.
 
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