To be honest, unless it's a low range GPU I wouldn't use a TX 650 for 2 cards. Get at least a 750.
people have run two 7870s on a 650 watt psu, i'm sure that two 6870s will run fine. its not always a possibility for people to upgrade an integral part of their system, and i can't spare the money for a new gfx card AND a new PSU. plus, it wouldn't make much sense since i will prolly upgrade next year or the year after to top of the line single-gpu solution and might as well upgrade my PSU then for a farther in the future crossfire/SLI solution.