Is a 700W PSU enough to crossfire 2 HD 7850

700 watts would have been really close on the 6000 HD GPUs. But the lower power consumption of the 7000 series should be fine with a 700 watt PSU.

HOWEVER, you must make sure, as others have stated, that the PSU is from a quality brand. I recommend sticking with Seasonic, Corsair, or Antec.