Checked out the link you last posted and saw a piece of helpful info that wasn't at other sites:
Minimum System Requirements
•Intel® or AMD® compatible motherboard
•At least 512MB system RAM
•A minimum 300W system power supply
•A PCI compliant motherboard
•50MB of available hard disk space
•CD-ROM or DVD-ROM drive
•Microsoft® Windows® 2000, XP
•VGA or DVI-I compatible monitor
You might be able to get by with 250watts, but as I said in a prior post I have tested the 8400GS on 300 watts, but I havent tested it on less. You may want to consider a PSU upgrade. And I'd rather buy new than used for both PSU as well as Video Card, there are lots of tired hardware on ebay and its a gamble. I have gotten burned on ebay before by items that were advertised as good working condition and are not, and the seller got your money and runs without ability to get refund or replacement.
A Video Card and a PSU on ebay may cost you close to $50 for both. Far more powerful computers can be purchased around the $80 mark for Core 2 Duo and motherboards that come with PCI Express slots and 300w PSU or better. So I agree with not investing much into this computer. If this case can take a motherboard replacement you could also use a modern motherboard's integrated video which is way better than the 8400GS in most cases.