Well, I'll be doing 5760x1200, so theoretically it'll be 1/3 as good as 1920x1200, but it won't always work out like that since with v-sync on, everything's getting capped at 60 fps, where as the card can easily push some games in excess of that.
I've tried some games across the two screens, which was annoying for FPS cause the crosshair's split right in the middle of the two, but L4D, Batman: AA and Dead Space all ran at flawless 60 fps, Dragon Age was also for the most part 60 fps... however Crysis was much less than 60 fps. :D