Im not too familiar with your psu but 400 watts should be enough as long as you arent doing overclocking and have too many other perpherals in your system. 450 Watt is recommended but you can get away with it.
You can always go for something lower than a 6770
NO! Your Diablotek PSU-shaped object will blow chunks on just about anything. It is not fit to power computer equipment. It might...might run a HD6670; problem is, it is such junk it may die anyway for no apparent reason (actually, the freezer in the basement cycled on), and when it does it is likely to take other parts with it. Add another $45 to your budget and pick up an Antec Earthwatts or Corsair Builder; then you can get a HD6770 or whatever will fit in your budget.