AFAIK, there isn't a performance boost just because they are dual GPU in one card, it is more of a convenience not having to Xfire/SLi.
Started typing about how the 590 and 580 SLi should perform the same, then I read this thread
But it really comes down to price, its $300 for another 5850 or $500 for a 580.
If it were me I would go with a single GTX 580, it performs on-par with 5850s in Xfire.
Would actually sell my 5850 + Corsair 850w and grab this combo
so I'd have enough juice to dual 580s later.
Please read through the thread thoroughly before responding.
Nobody wants to read a wall of text, formatting is your friend.
i5 3570k | 8GB G.Skill 1600MHz | R9 290 | SeaSonic 750X