If they aren't overvolted... I'd say "yes, but".
The AX850 is a pretty good PSU, just look at JGs review
OCed 5870 at stock V is 200-220W, like 180 with mem @ 300
OCed 5970 at stock V is ~300W, about 270 with mem @ 300
Stock wattage is ~ the same as OCed with underclocked mem.
So that's < 700W load on a PSU rated for 840W continous on 12V.
now, theres the "but" part... simple, I wouldn't want to run a PSU at > 80% rated 24/7 for months.