20A on 12V rail?
That's pretty low for 500W PSU.
What is your PSU? brand? model? how old?
Its a "allied" I got it about 4 years ago used it many hours a day
I figure its too old for this system thats why it causes it to shut down once i load far cry 2 even on the onboard video
They still sell the power supply online for 69.99