My opiinion is, to use a good single card whenever it will do the job.
SLI is a bit more complicated, and requires a stronger psu.
The benchmarks will usually be good, but some games do not scale that well with sli.
Also, get a 2500K instead of a 2600K. It will save you $100 that can be applied to a stronger graphics card.
The 2500K will OC to the same limit as a 2600K, and the extra hyperthreads are not useful in games.
I would definately get the two cards for gaming, if you were to get a single stronger one (like a 570/580) then it would be different, but between the two options you are giving...
Some games may not scale very well, but still it should most of the times be better than a single card, and on the games that it scales as it should (most of the games) you will really see the difference.