Crysis 2 is going to be less demanding than the original Crysis.
The best advice I can give if you are not sure is to buy a motherboard that is either SLI or XFire capable, then sticking on card of your choice. If you think Crysis 2 or any other game is not performing as you would like it to then stick in another card.
I have not seen reviews for the GTX 560, but here's on of the the GTX 570. As you can see it edges out the HD 6870 by a little, so the GTX 560 is likely to be a little slower than the HD 6870.
Since the GTX 560 is a slightly slower version of the GTX 570 it will likely use a little less power, but the HD 6870 will use even less.
Here's another perspective about power consumption from Xbitlabs:
Notice the huge jump in power consumption from the HD 6870 to the GeForce GTX family?