Welcome to the forums.
The Wattage of your PSU is of some importance, but equally or more importance is the strength and stability of the rails (3.3 volt, 5 volt, and 12 volt). You can read the strength (amps) straight from the sticker that's on the PSU. Stability can be observed by using programs such as Speedfan and Everest. If the voltages vary minimally (+/- 3% or so), the rails are considered stable.
Barring very weak or unstable rails, a decent quality 400 Watt PSU is likely powerful enough for an x800gt. It would be helpful to know what other components you have in your system, though. Please take a few minutes to fill in your system specs by clicking on the User CP link on the upper left corner of every page.