R9 280X can draw power from PCI-E (about 75W) and from 6pin + 8pin (175W) or 8pin +8pin (200W), so max consumption is somewhere around 250W.
You can see consumption when mining:
let me tell you my system configuration for this rig
Motherboard: Asrock 990fx Extreme
Cpu : Amd Athlon x2
Ram : 4GB DDR3 @1600
HDD : Sata 250GB
All this powered with a 550Watt Heroichi power Supply.
GPU's : 1x Asus R9-280X DirectCU II TOP
http://www.guru3d.com/articles-pages/asus-radeon-r9-280x-top-review,8.html Power consumption Radeon R9-280X DirectCU II TOP
System in IDLE = 119W
System Wattage with GPU in FULL Stress =
352W Difference (GPU load) = 231W
Add average IDLE wattage ~10W
Subjective obtained GPU power consumption = ~
243 Watts 1x Asus Nvidia GTX550Ti
http://www.bit-tech.net/hardware/graphics/2011/03/15/nvidia-geforce-gtx-550-ti-1gb-review/10 System Wattage with GPU in FULL Stress =
248WGPU's powered with a 650Watt Heroichi power supply. Using a 550w supply system restarts. So here is my proof that i'm not making assumptions.
Both power supply's are gold plus 80.