Well it depends on how you advertise it. If you put in on craigslist for $400 you're never going to get a single e-mail of interest. If you say $400 oBo you'll get a lot of people offering around $200. It looks like it's a decent enough computer for mid gaming (although you would need to put in a new graphics card and update the OS to Windows 7 at least. Right there, $300 in upgrades, minimum). BUT it looks like you have a good starting kit if you wanted to build your own Hackintosh (Not sure if that's entirely legal to say here, so I am not suggesting you should use it as a key selling point, just saying it's probably possible.)
If it was me, I'd wipe out the drive and then Zero it out and put Ubuntu on it. Someone could use it as a low level server or even a torrent box. I'd say list it for $500 obo and you'll get about $300. Good luck!