If you go with a 570, here is something to look at:
Newegg.com - Computer Parts, PC Components, Laptop Computers, LED LCD TV, Digital Cameras and more!
With that combo, I don't think the 580 is worth the extra money $120.
About 560 SLI, vs 580, it is generally suggest that you go with the fastest single card solution you can get, then by a second whenever you need more performance. The reason is because if you get two slower cads now, when you need to upgrade, you have to sell your old ones. With a single card you can just buy a second. Really it is just about being able to upgrade later on, for cheaper.