It's really the amps that matter most for a PSU.

Not sure if the 400 watt is especially accurate (may require more) BUT..... I am absolutely sure that the 300 will not be able to stand up to the pressure.

A little info for you to look at: http://www.xoxide.com/buy-computer-power-supplies.html

Fix the PSU and possibly grab the 640 meg version of the 8800. I wish I had that choice when I was buying my parts. Every shop in Singapore was basically out of stock of the 640 meg ones.
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†Jesus†

The nVidia Geforce 8800 GTS 320 MB is a great video card with a great price tag. However, if you desire to run programs at high resolutions like the one you mentioned, get the 640 MB.