A 12v is not used only for gfx card it is also used to power other things!
Even recommended psu for gtx 660 is 450w with minimum 24A on a single rail.
First thats stupid/dumb. The first (?) ATX spec that allowed multiple rails capped each rail at 18A. Most stay there or go up as high as 22A. While not impossible most multi rail 12V rails aren't 24A. Second, you don't need that much power from one rail. You'll have no issues if some of the power comes from more then one rail. A card has no issues taking 75W from the motherboard while taking the rest of what it needs from another rail. They do it all the time. Finally third, most "recommendations" you'll see are companies trying to "CYA" or they are worried you are going to use a POS like a Diablotek PSU. So they overstate what you really need.
If you have a 150W GPU (GTX660) and a 125W CPU (FX 8350) your power draw is ~325W. Any PSU that can output that power on its 12V rail(s) will work. You'll obviously want a bit more. Most good 500W PSUs can do 408W+ so they are fine. But there are new gold rated 450W units that will handle this load with ease.