List all of the components you have in the computer. i7-950+ Board, how many RAM sticks? How many hard drives and optical drives? Do you have a stock cooler on the CPU or large aftermarket one? How many fans? Just add everything up and make sure the PSU has at least 100w extra for breathing room.
Generally speaking, 750w is enough to handle those 2 video cards plus normal computer setup.
For more information for estimating your power draw, you can try a PSU calculator like the one from Cooler Master.