ACTechy :
The CX750m wil be plenty for two R9 280x, no problem (especially if you don't want to spend a boat load of cash).
An overclocked 280X can pull 300 watts all by itself. Not the whole system just the card. You want to risk a system that can potentially pull 600 watts just with the graphics cards to a budget CX750 with cheap Chinese capacitors?
I hate this trend of it's a Corsair it's fine for anything with the CX/CXM. They are budget units and should not be stressed.
If it was my system I would trust RealhardtechX. They maintain the most comprehensive PSU database on the internet and can be trusted. I would go with a 900w unit.
You do realise two gtx 780's pull around 500w right? 750w will be fine as long as you don't have decide it's smart to put something like 1.5v on your cpu.
Getting a good 750w is a different story entirely though, as a rule of thumb you should probably stick with 80plus gold or higher
I've heard good things about fsp's aurum pro's, but things like seasonic, corsair, etc are always good choices.