Well, really it's all a matter of personal preference. Most SLI configurations are too poweful, by which I mean you have so much processing power that it's overkill. So you can either build SLI to be the biggest baddest kid on the block, or you can build it to extend the life of your existing card, or to try to build a really budget based build.
Or you can just buy a high end single card, and be perfectly happy. And if you're not comfortable with ripping apart your computer, this is the route I would suggest. You can run just about any game at max settings on 560 and it will run great. In a couple years there might be a couple of new games that you won't be able to max out, and will have to turn down to get good rates, and you can either live with it, or upgrade at that point. Doesn't sound so bad really. Possibly in that time you might have gotten familiar enough with comps to swap out your mobo and just drop in another 560 too.
So for now I'd say just enjoy whacha got.