In everything apart from crysis you should be pretty much fine to max settings on that monitor with only occaisional problems, crysis you should be able to get decent performance out of with only a few settings set below very high, going SLI, MIGHT overcome those issues. SLI can bring issues of their own which can detract from the visuals.
If you have a power supply already to run two cards then it might be worthwhile. As you have the 260 not the 280 Im assuming you dont have unlimited amounts of cash to throw at a rig in the hopes of gaining 10% more fps? Decent power supplies to run two high end cards dont come cheap.