On my Gainward GeForce GTX 650 1GB it says "Graphics card require: Minimum 400W or greater system power supply (with a minimum 12V current rating of 20A)"
My old Chieftec cft 560 a12c has an output of 560w total and three 12v lines. 12V1=14A, 12V2=23A, 12V3=15A, and a combined output of 440w on all three lines. Will it do the jobb?
If you can find out which cables assigned to which 12V rails, it will likely work. the key is to use one of the 12V for CPU supply, the 2nd for motherboard & drives, and the last 12V for the GPU power connector.
Note that PSU degrades over time: MY 650W Enermax was able to run triple cross-fire @ 720W when it's new, and now struggles to run 400W system @ 8 years old.