If you only plan on running a single HD6770 then a quality 400 watt power supply would be fine. You could actually get away with a 350w power supply. If you ever plan on upgrading the graphics card you can run just about any single graphics card setup with 550 watts.
Stick with Corsair, Seasonic, PC Power and Cooling, XFX, Enermax, Silverstone, OCZ and Antec for quaity.
its a 400w psu so what do u say will it handle the pc well ?
17 amps on a single 12v rail
25 amps on the 3.3v
32 amps on the 5v
It's an old design that is no where near the equivalent of a modern 400 watt psu, think of it as a 250 watt psu and you should be safe.
Better yet, try and find a decent quality 400 watt replacement.