That should definitely work. What are the other specs of your computer? An 850W power supply could easily provide the power needed to your PC.
How do u no the problame was from psu ?
Usually,bad quality PSUs cannot handle GPUs like your R9 290X,like your Corsair 750W
They can't handle the power they say they can.Your Corsair 750W may say it can handle 750W,but in reality,it can only handle up to 500W. 😉
The R9 290X consumes 300W under full load.With your entire system,it consumes up to 450-500W.
That's why your PSU was the problem.
Get that EVGA Supernova G2 1000W and you should be able to boot your system.It also supports Crossfire and SLI,so if you ever wanted,you can stick another R9 290X if you want. 😉
All the best. 😉
^Adding to that:
The PSU you currently own has LTEC capacitors of inferior quality. They're good as long as they're not utilized more than 65-75% of their capacity most of the time. They can support average rigs but a R9 290X or a high end rig should always be complimented with a superior PSU of atleast Tier 2 Class A. Tier 1 is the best option for any power rig.
PSUs are one of the main reasons for such blackouts so its relatively easy to relate that symptom here too. I hope it helped