And on the r9 280x box they wrote minimum 750w supply becAuse they take into account the poor low quality supply that do not provide good and stable amount of current to the system so no need to woory with that 620 u can even oc your card to 1100 at core and 1700 on memory
I personally hAve 2x r9 280x (1 vapor x and other xfx black edition )both at the above stated clock extremely stable with cooler master silent pro m 850w supply