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 = 248W

GPU'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.