Radeon 7990 vs GTX 770 or Dual GPU vs Single GPU

Hi!
I am thinking of to buy an I7 5960x on release.
But the first step would be buying a new graphic card.

Did i have some advantages with a dual-GPU card?
Afaik WOW supports only 1 gpu. What happens with more then 1 WOW client? Can i assign a gpu to specific wowclients?
I ogle with the old radeon 7990 6gb.
Or should i buy a GTX 770 4GB?

I found some post where users advise single GPU for multiboxing, but i couldnt find the theory behind this advise.

regards

I use two 770's in sli with 2 monitors to 5 box. Full screen stacked with the video feeds on the second screen. The sli works very well compared to a single card. A few times I've noticed some stuttering and it was due to the sli being turned off.