I would change your mind set around buying a performance video card. I would go for a powerful single card if that satisfies your monitor setup needs. SLI does not work with all games and can have some weird issues. I was in the same boat as you when I built a new machine and I wish I had just went with one single powerful card. Think of SLI when you want to upgrade your system down the road for an SLI setup.
I also like what Mr Alpha mentioned. You will get performance like a single 580 only when SLI is working perfectly. Otherwise, you are stuck with a single 560 performance level.
Take it from me. I built a 460 SLI setup instead of a single 480 which was the previous Nvidia lineup. I wish I would have just went with a single card. SLI is fun, when it works.