I tend to box them up in their package and hold them for posterities sake, if they are worth any emotional/milestone(ish) value. If not and the question of funds comes in, I tend to sell them off on a e-marketplace or with the aid of a friend who works at the computer store.
It's not a bad idea to hold onto parts for troubleshooting sake. It also helps to hold onto some parts, if you want to start a PC repair business.