580 GTXs and dual boxing.

[Ualaa]

I'd guess, there will be some who upgrade from a card older then a 480, and pick the 580.
Don't see too many people moving from 480 to 580.

There will likely be reviews, comparing the new card to existing cards, in a variety of situations.
That's your best bet.

[Noids]

Not sure about boxing comparisons, but tomshardware has a fairly extensive comparison. Looks like a fairly decent jump in performance as well as reduction in noise over the 480. What this will translate to in terms of WoW performance is anyones guess.