I personally have not tried two cards on FFXIV. My understanding is that the processor plays a large role in FFXIV and that the sheer muscle of the GPU won't get you as far as one would hope.

This may be just me, but I think I would go for the GTX580 over the dual GTX560 in the long run. Should I ever decide to go completely spend-happy-crazy and dual GTX580s.

But for either route, you'll also want to bump up the power supply from whatever it's currently on to maybe an 850W or higher.