Unless you have the latest and greatest cards, it is usually more cost effective and efficient to avoid SLI and crossfire.

Whether or not it's "Worth it" really depends on the prices. I notice an extreme difference between one GTS 250 and two in SLI. You should notice the difference in FFXIV right off the bat.

3 is too many..

Also, no.. two 460s cannot run this game at max settings efficiently. But that's mostly because of ambient occlusion.