This PSU Website gives a good advice on the minimum PSU you need for your system to run varius GPU Cards: PSU REQUIREMENTS - RealHardTechX 
I would recommend a PSU of at least 750 Watts that is Metal rated (Platinum, Gold, Silver, or Bronze).
But according to CNET Specs on the Sapphire R9 280X Dual-X GPU card you need a minimum of 750 Watt PSU: Sapphire RADEON R9 280X Dual-X OC - graphics card - Radeon R9 280X - 3 GB Specs - CNET
In which case I would then recommend a PSU of 850 Watts - Metal rated.
