He relays the technique of retaining one's identity after death to Obi-Wan and Yoda, reflects on the mistake he made with Anakin, and then fades into the Force forever. Very bittersweet if you ask me.
As for Tarkin, I see where you're coming from (and I agree with characters such as Mon Mothma), but Tarkin is the #2 guy in the Empire in ANH. That warrants some sort of backstory in RotS.