I play on dual GTX660s and while there is a minimal difference in fps in instances and sparsely populated areas, I see an improvement of about 6-12 fps in heavily populated areas when in SLI than when in single gpu mode. Now, with two 780 Ti cards, you will probably see no difference in FFXIV. The game is heavily CPU bound and is still only using the DX9 API. There *might* be a bump in fps with the switch to the DX11 API, but it will be negligible on such high-end graphics hardware. FFXIV should not be your reason for buying a second 780 Ti.