I would like to upgrade my power supply, in my HP Pavilion a630n, which has a 300 watt unit from factory.So I can upgrade to a good graphics card. I have a 8500GT with passive cooling right now, I would like to upgrade to a 8800GT if Possible.
Thanks John

My Question, How do I know my computer can take it ?

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).