That psu has many problems. First it can't even get close to 42a on the 12v rail, it uses very low quality components and is based on an antiquated design. In most places this psu was discontinued long ago so it has probably been sitting on a shelf for awhile. Here is a tier list get something 550w or larger from tier 1 or 2(a or b). http://www.tomshardware.com/forum/id-1804779/power-supp...
You need a new psu, 400w is not enough. Plus it most likely lacks the proper power connections for the card, which is the reason you are getting the error. Get a good quality psu with atleast 42a(502w) on the 12v rail.
Thermal and Power Specs:
98 CMaximum GPU Temperature (in C)
230 WGraphics Card Power (W)
600 WMinimum Recommended System Power (W)
Supplementary Power Connectors One 8-pin and one 6-pin
See where it says the power requirements? Your 400 watt PSU is too small. Do you have the one 8 pin and one 6 pin power connector running to the 770?