I don't know about Crysis 2, but Crysis 1 it'll be iffy. I have overclocked crossfire 5850s and I get around 50+fps in Crysis 1 at 1080p and max settings 2xAA (Although in Warhead, max settings and 4xAA runs at 60fps). My CF 5850s are definitely faster than a single 560...
But if you want to know for sure, just read some reviews. Crysis is almost always a benchmark.