can take what? the power supply? well, is there enough room in the case? that's the only real prerequisite for upgrading a power supply. if you are wondering how much power you'll need for the gpu, use the link i supplied to calculate how much power you'll need to run the 8800gt.
if a component inside your computer is rated to use a specific amount of power, it will use that specific amount of power and that is it. if you psu generates extra power, it will simply hold that power in reserve until it's needed. upgrading your psu will not cause your computer to go up in smoke (if you install it correctly).