The most your 6770 GPU will use is 108 watts; your CPU 95watts.
The recommend PSU(power supply) for you system is 450 watts(with one 6 pin PCIe connector available).
Nothing else uses much power: drives, ram, fan ect...
The overkill is so your PSU is never loaded at 100%; 108+95... you'll never even be at 50% of 450.
You could get a 600 watt if you think you might go crossfire(two 6770's) in the future.
450+108=558 and then round up to 600 for the safety margin. And again, that's what AMD(ATI Radeon) recommends. Also of note: the 6770 is a direct refresh/name-change of the 5770. You can even crossfire them.