I recently built a computer and bought 2 do 280x gpus, but have had a nightmare with the system. I'll start from the top:
Fx990UD Gigabyte MoBo
8370 Vishera 4ghz cpu
8g 1866 Mushkin Red line ram
2x R9 280x Gigabyte OC edition GPUs
750w Thermaltake 80+ Gold rated PSU
Zalman Z11 mid atx case...