I suggest sticking with 1 card, and someone in the ISBoxer Chat Room said a day or two ago that they got the new 580GTX and have not had any problems with it. That person is currently playing WoW in Direct3D11 mode (with the dev build of IS).

About using multiple video cards, read this for some basic info: http://isboxer.com/index.php/guides/...le-video-cards. I recommend sticking with 1 card because, as the link outlines, you may be opening yourself up for performance hits if you're intending to swap windows between the monitors. Using 1 video card will not incur this penalty and should perform just fine (especially if it's a high-end video card). The 5970 counts as 2 cards for the purposes of this paragraph.