It's the GPU that sucks the most juice more than any other single component.
For the GIGABYTE GV-N770OC-2GD, the manufacturer recommends a 600 watts PSU.
Don't get any old PSU though, you need a reliable and trusted brand like Seasonic, Corsair or Antec.
Actually, the GPU maker's recommendation is usually overkill but best to to play it safe and get 600 watts one anyway.
Most enthusiasts would say 500 watts PSU is plenty but, well, it's your call.