that's Crossfirex bro... multiple GPU term for AMD...
a 500W true rated PSU is enough to provide juice for the whole system... if you'll do OC... mura'g gamay2x na'g head room ang 500W if mag furmark... hehehe... kun games lang @ full load, below 300W ang power consumption sa R9 280x... if furmark or synthetic bench mark, dako kaayo @ full load... nearing the 400 W na sya for the GPU alone...
for benchmark...
gigabyte R9 280x OC... kani, card power consumption ni... higop sa kuryente using Furmark... the same sila lamoy sa HD 7990... but in gaming, 216 ~ 244W lang ang card power consumption in playing Crysis 2 @ 1080p on Extreme profile...
Gigabyte R9 280X OC 3 GB Review | techPowerUp
here's for Sapphire's Toxic... kani, measure ni sa system... on the wall power drain using Unigine's Heaven Benchmark version 4.0...
Sapphire R9 280X Toxic Review » Page 16 - Sapphire R9 280X Toxic Testing: Power Consumption - Overclockers Club