Newbbuilder11 :
Since you are running only one, yes. Two of those require a 1000k PSU
Whoa a 1000 kilowatt PSU!
On a more serious note, the R9 295X2 has a max TDP of 500W. So yes that 850W PSU will run it just fine.
To run two of those cards, you would need a 1200W PSU. But that would be a stupid thing to do.