Darth Vader: I find your lack of faith disturbing.
[Princess Leia gets her first look at the Millenium Falcon.]
Princess Leia: You came in that thing? You're braver than I thought.
Han Solo: Hokey religions and ancient weapons are no match for a good blaster at your side, kid.
Ben Kenobi: Obi-Wan Kenobi? Obi-Wan. Now there's a name I've not heard in a long, long time. A long time.
Luke Skywalker: You do know him then?
Ben Kenobi: Of course I know him...he's me. I haven't gone by the name Obi-Wan since, oh, before you were born.
Luke Skywalker: I want to learn the ways of the force and become a Jedi, like my father.
Princess Leia: Help me, Obi-Wan Kenobi. You're my only hope.
Darth Vader: Your powers are weak, old man.
Obi-Wan: You can't win, Darth. If you strike me down, I shall become more powerful than you can possibly imagine.
Darth Vader: I have been waiting for you, Obi-Wan. We meet again at last. The circle is now complete. When I left you, I was a learner. Now I am the master.
Obi-Wan: Only a master of evil, Darth.
Princess Leia: Aren't you a little short for a stormtrooper?
Han Solo: That was luck.
Ben Kenobi: In my experience, there's no such thing as luck.
Luke: I have a very bad feeling about this.
Obi-Wan Kenobi: Who's more foolish, the fool or the fool who follows him?
Darth Vader: The Force is strong with this one.
Obi-Wan Kenobi: The Force will be with you. Always.
C-3PO: We seem to be made to suffer. It's our lot in life.
Han Solo: You're all clear, kid! Now, let's blow this thing and go home!
Luke Skywalker: Are you all right? What's wrong?
Obi-Wan Kenobi: I felt a great disturbance in the Force. As if millions of voices suddenly cried out in terror and were suddenly silenced. I fear something terrible has happened.
Luke Skywalker: Look, he's heading for that small moon.
Han Solo: I think I can get him before he gets there. He's almost in range.
Obi-Wan: That's no moon. It's a space station.
Answer: It is heavily implied that one who becomes a Force-Ghost achieves an untold level of power upon entering the state. Given that they have become a pure entity of the Force, it seems to back up the statement. Obi-Wan also becomes free to assist Luke in any case.
Darius Angel
In addition to this answer, I think Obi-wan also became powerful because he "let go." He did not feel a need to defeat Vader on this occasion - he was willing to surrender the fight and "move on" to a new state. Something that Vader might not currently understand.