And then you have to deal with SLi drivers (not that they're bad, they just aren't as solid as single gpu), and micro-stuttering. Not all games suport SLi either (most do, but there are a few).
So 2 x 470's = $520, TDP = 430W
1x 580 = $500, TDP = 244W
The cost of the 470's is 104% compared to that of the 580. The performance is about 125% that of the 580. The power consumption is about 176% that of the 580. The 470 setup will also be a lot hotter and a lot louder. If you get the single 580, you can always add another one later on. If you go with the 470's your upgrade path is limited to tri-sli (if you have a tri-sli supported board), which doesn't scale so great, and if your monitor setup requires that much GPU horsepower, you will also need a lot more VRAM than a 470 offers.
Personally, I think the 580 is a much better choice. Now, if I had 2 470's, would I replace them with a 580? No. But if I was just now making a decision, 580 all the way.