For a R9 280 you need a 550W power supply according to Real Hard Tech.
You can work it out yourself with a Wattage calculator eg http://extreme.outervision.com/psucalculatorlite.jsp
For a R9 280 I'd recommend Seasonic 520, XFX 550 or Antec HCG 520M. They are all excellent power supplies.
Even the 550W recommendation will carry a safety factor - in case you want to buy a real crappy power supply.