Single card is good too. The real luxury of a well configured crossfire system, is you have a card, if one fails. Or you have flexibility to sell one, and apply it to some kind of upgrade.
The question wasn't as much about what is the smartest design(which varies for the users knowledge and situation). The question was more about what parts to work within the budget. $560 is tough, because you're right below a FGPA price range in the same has territory, and under a single card setup that would get you a similar hashrate.
Just because you have two cards, doesn't mean they are in crossfire. Hense, my question. Is there any benefit to having them in crossfire? My understanding is that only helps out with framerates on a single monitor. (If you run dual monitors, I think its better to not run them in crossfire).