what is the power supply required for r9 280

A quality 550-600 watt power supply(modern power supplies have a large portion of the overall wattage dedicated to the 12 volt rail.[12(voltage) x amp = watts for the 12 volt rail]) would have no issues with a modern system running a R9 280. Having extra power never hurts.

The system configuration plays a roll. Lower powered parts can get by with less power, but a general rule is 550 is enough for a normal system(even overclocked) that would be paired with that card.