Each ASUS Radeon HD 7970 DirectCU II GPU is going to use, at a maximum, of around 300W each. So using two of them in CrossFire is going to use around 600W just for the GPUs - without taking into consideration the rest of your setup.
I would suggest something like an 850W or even 1000W PSU to be safe. You won't need, or require that much power - but it's always better to have more than not enough. Any brand name PSU would do, but I'm a big fan of Corsair's range of PSUs, so you could go for something like the Corsair AX860, or the HX1050.
If I had to make the decision for you - I'd go with the AX1200, only because I'd want to cover myself for future upgrades. That way, you are pretty much future-proof for any next-gen GPU action.