I seriously hope you do not take Phaedrus advice as gospel, Honestly he has just told you a 400W supply will be ok! He hs not asked what your entire system will be comprised of, therefore has no Idea at all what PSU you need!
Besides that, a 6950 needshttp://www.guru3d.com/article/radeon-6950-6970-review/11
Scroll down page, 6950 = 500W minimum 6970 550w minimun.
Now if you are planning on Unlocking that 6950 with 6970 bios, then yes Minimum is 550w, but No-one in thier right mind goes with the bare minimum in PSU, simply the most important part of your PC, the part that if it fails, in best case scenario stops your pc working till you replace it, and in worst case scenario, takes out your CPU/Mobo?Gpu along with it when it goes POP!
And then saying 700w would be OVERKILL for Crossfire 6950, OMG, that would be the bare minimum i would expect to see!
Wolfeman, your Corsair 650w is a great choice, pefect for stable singel 6950/6970 configuration, It will also allow you to upgrade that card in the future as well when they come out!
If you want Crossfire, get the 750W model or greater!