Do I need SLI with this monitor?

Whether or not you need sli depends on which graphics card you are running!

I run games at 2048x1536 regularly which is more taxing, my single 4870 is just about adequate for the vast majority of games, my 8800gtx before that struggled with quite a few. I have to drop the AA or resolution (which I can do nicely as its a CRT) if I want to run crysis at very high. just plain high I will have to test when I get home see how that performs with a single 4870 (comparable with a single 260 for performance). I seem to remember the jump from high to very high made a huge difference to performance. But I believe you can get further by tweaking individual settings and leaving many on very high and only turning down a few to high. I was prepared to put up with the odd stutter for the sake of seeing crysis at its best.

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.