Just look at from very general terms.
SLI is built for what? Speed. Speed Speed Speed. Its only made so you can process those games with that extra 25 FPS. At current, the only thing they actually ENABLE you to do (over the same card by itself) is EXTREMELY high res gaming and as someone else mentioned it allows you to run HDR and.
Now look= ahead on the couple years... or year... or hell, even the next few months. What do you see? Dx10. Whats this mean? It means your card needs COMPATIBILITY. Compatibility Compatibility Compatibility. Dx10 cards are all ready by themselves gonna be RIDICULOUSLY fast for their time, just as the 79xx's were. The next age in graphics cards will all be about how cards handle Dx10, now how FAST they can handle dx10. That will come much, much later.
So, if you can't get the preformance you want from 79xx cards, just OC it. If you're still not getting what you need, and you're playing on extremely high res, then go SLI for a cheaper cards, such as 7900/7950GT. But unless you're one of those freaks who has a 300$ monitor, SLI is not, and probably never will be, for you.