A Radeon R9 280X is a rebranded Radeon HD 7970 GHz Edition.
For two Radeon HD 7970 GHz Edition graphics cards in 2-way CrossFire mode AMD specifies that a minimum of an 850 Watt or greater system power supply should be used.
Sapphire Technology and H.I.S. both specifiy a 1000 Watt power supply for two Radeon R9 280X graphics cards in 2-way CrossFire mode.
Under power your rig at your own peril.
But, on eXtreme power supply calculator, it says a minimum of around 600 Watts. And I put all my specs. the CPU, GPU, RAM, MOBO, HDD, SSD, and it says 600 watts. And the Corsair RM 750 has 750W and enough amps...