They do exadurate a little when it comes to the minimum
PSU requirements, as they need to account for all the possible hardware you may have in your PC.
It's doesn't look like you have much in your PC in terms of hardware, although I can't tell what
CPU you have there. I would say it would work quite happily on that PC with a 280w
PSU. You can double check here:
eXtreme Outer Vision - eXtreme tools for computer enthusiasts (click go Lite, and enter the hardware details of your PC, including the 8400GS to find out your requirements).