Let me give you more accurate information. Each OEM has different recommended minimum requirements for their cards. As for as the R9 280 GPU.. here is a quick breakdown for you:
Powercolor & XFX DoubleD : 750W PSU Minimum
Gigabyte (GV-R928WF3OC): 500W PSU Minimum
Sapphire DualX (R9 280 100373: 600W PSU Minimum
So unless you buy a new PSU .. only the Gigabyte model should be used with a 500W unit. If your PSU is more than a couple of years old then you should go ahead and get a new PSU.