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.