One GTX 780 Ti doesn't compare at all with two HD 7950s in Crossfire. Two HD 7950s in crossfire crush a single GTX 780 Ti. It's not even close so it would be a huge downgrade.
BONUS: nVidia.com says that the card draws 250W and the minimum recommended PSU for one is 600W so add 250W to that and you hit 850W. You should be alright.nvidia.com GeForce GTX 780 Ti Specifications
I caution you however that your current setup is already so powerful that it can play ANY game at max settings with no problems. Changing your GPU setup would be a complete waste of money at this time.