it will work well with or withought OC to your cpu your PSU can generate 850 watt if needed like turbo boost or OC itself my TX 750 watt can handle the rig you can see in my signature and your 6950's are better than my 5870's because they consume less power
You ordered, but for the future I would have recommended an 850W for sure.
FYI, all PSU's are different. That single WATTAGE rating is calculated by adding up all the maximum power outputs for each voltage output.
The graphics cards, for example, require enough power on their specific power cables. Often the CURRENT is listed for the graphics card section.
It gets confusing fast, but there are many websites and PSU calculation tools which would help clear this up.
While I think the TX650 will work I think you're pushing it. You may need to be careful about overclocking your system as that can significantly increase the power requirement.
If you think your system appears unstable, simply shut down and remove one of the cards and see what that does. If stability is now normal, you need a higher wattage PSU specifically designed to supply the proper power for both cards.
I love learning i would love to check out some of those websites to calculate and learn the differences between single and double rail...I read about them but I dont know the differences. Thanks for the help.