A single 570 now would give you the option to SLi later perhaps and unlike bystander I have found that most of the games that I have played over the last six years or so have utilized SLi.
Most games I have played also support SLI, but not all. Some just performed like a single card, some have added glitches. When picking a card to start with, a single card is usually best, unless you are already at the high end and want more power.
And like you said, you can SLI later down the road.