In general a 400w PSU should be able to handle an OC'ed Radeon HD 7770. The max power draw will be in the low 90's watts. A stock Radeon HD 7770 maxes out at 83w.
What CPU are you using? AMD CPUs uses more power than Intel CPUs. For example under stress, the FX-8350 uses more than 100w of power than an i5-4670k under full stress.
I use AMD athlon ii x3 450 though when i used something called watt calculator on the internet it told me that my system should draw little more than 300 w