Quote from: ayayay on January 12, 2014, 04:46:41 PM

so if I add a Platinum 850W+ ATX12V / EPS12V 80 PLUS PLATINUM Certified psu can I then run 2x r9 290's

For a 850w, you can certainly do 2x 290. but make sure you get a good one like corsair or seasonic. It is easy to put the numbers on there but the actual performance is way below.

Also, 2x 290 will take 600w at least, and adding another 150w for the rest of the stuff. you are looking at 88% load for 24x7.
efficiency wise, neither good or bad for a platinum(but again, make sure it is indeed works like a platinum).
But, PSU will age and much quicker when you constantly put high load on it. When this happens, the rated 850w will becomes lower.
Usually after a year of heavy uses, you can expect about 15% loss of that rated power.
So I would get a 1000w gold for 2x 290 and the system.