Me? If the computer is still in working condition, I would keep it in case of an emergency. When something goes wrong with a newer computer and it needs to be repaired, I take out the old one that I could use until the new one is fixed. Only disadvantage is that the tech is, of course, outdated and may not be useful today.
I try to keep two computers up and running at all times that can both do what I want. Right now my main is a Core 2 Q6600, GTX 460 1GB, 2x2GB of DDR2 800 RAM, and 1TB + 320GB hard drives, while my backup machine is a Phenom II X3 720 BE, 9800GT, 4x1GB of DDR2 800 RAM, and a 500GB hard drive. If something happens to my main the backup machine is still good enough to do most things I want.
Anything older than that I generally give away or sell for a pittance, usually to friends that have kids so they have some sort of viable desktop to use.