It's said on the XFX package, at least 500W with two 75W 6pins, and 600W with four 6 pin for dual mode CSF.
I going to get 550W one for CSF. is this going to be enough or less effective?
More about4770 dual slot power supply requirement
To my knowledge, each 4770 pulls 80w max. So 550w seems like more than enough so long as you've got more than a single 12v rail. So long as you dont have a ridiculous overclock on your CPU, and/or loads of other peripherals.