Well, when a jedi dies, he actually lives on in the force. The jedi must probably be strong in the force if he wants to be visible for non-jedi.
And for sith...its probably the power of the force is all that matters.
And yes, the can posses a jedi/sith, they can even kill one.
*sigh* that sure is discouraging.... EU is Expanded Universe..... like the message board category you were in when you posted that. & a good example of a jedi/sith ghost would be Ajunta Pall, in the game "KOTOR" which is Knights of the Old Republic (for any other newbie's lurking in the shadows)