Similar issue: I have a X800 AGP GTO that I just replaced with a GTS 250 1 GB PCI-E and after searching on E-Bay realized I wouldn't get much for it so I'll just keep it. Look to see what you'd get for it and if it's peanuts just keep it.
Yup, just sell it. There are people out there with more faith in Physx than you have (if you had faith, you wouldn't be looking to sell), so sell it to them and make them happy