Well hoestly it comes down to budget, a 560 is a great card for sure and I would hate for you to buy a 6870 and then find in this or that game you're not getting the performance you want. That said, you generally don't need max FPS when not playing shooters so a 6870 would probably be adequate.
Also when you build the new PC, you might consider crossfire which would be a lot cheaper than SLI 560s obviously.
So... eh it's really up to you. I'd probably just bite the bullet and get the 560.