If you measure by raw FPS, the 2x 570s will win out over the 580.
However, for $500 you can do a Powercolor 6870x2 (2 processors on one card) + random 6870 and you should be able to beat either of those options.
The 2x 570s could potentially generate a lot of micro stuttering which could detract from your gaming experience.
Read this article if you want to hear more about some different things that play into this.http://www.tomshardware.co.uk/radeon-geforce-stutter-crossfire,review-32256-15.html
That talks about micro stuttering, the 6870x2 + 6870 and other stuff like this.