Trivia: Mark Hamill annoyed Alec Guinness so much on set (asking career and acting advice) that Alec once paid him 20 pounds to go away.
Trivia: The film's screen tests were spoofed in a 1997 episode of "Saturday Night Live," which included Kevin Spacey playing Christopher Walken auditioning for the role of Han Solo. Walken really was considered for that role before Harrison Ford was chosen. Spacey also played Walter Matthau auditioning for Obi-Wan Kenobi and Jack Lemmon auditioning for Chewbacca.
Trivia: Billy Dee Williams, who later played Lando Calrissian in "The Empire Strikes Back" and "Return of the Jedi," auditioned for the role of Han Solo.
Trivia: The sound that is made when they power up the Death Star's hyperlaser to destroy Alderaan is the same sound used in the THX Certifier on many DVDs.
Trivia: During the trash compactor scene, Mark Hamill (Luke Skywalker) held his breath so long that he broke a blood vessel in his face. That's why many of the shots show only one side of his face.
Trivia: When Obi-Wan Kenobi is deactivating the tractor beam, the set Alec Guinness was on was only six feet above the floor.
Trivia: Luke Skywalker's name was originally going to be Dirk Starkiller.
Trivia: In the Death Star scenes (yes, all of them), whenever the Imperials walk, you can hear their footsteps. But when Tarkin walks, you can't hear him. This is because Peter Cushing (Tarkin) found his Imperial boots so uncomfortable, he didn't wear them. He wore carpet slippers, so you can't hear him. The only shots in which he can be seen wearing boots are shots in which he remains stationary.
Trivia: According to the documentary "Empire of Dreams," as well as interviews on the "Revenge of the Sith" DVD, George Lucas originally wrote the "Star Wars" trilogy as one long serial titled The Tragedy of Darth Vader, which ran about 200 pages. He then broke it into three separate stories, and focused solely on the first part, which became "Star Wars."
Trivia: The heavy rifle used by Chewie in the Death Star is a German WW2 MG 34.
Trivia: Peter Mayhew worked as an orderly in a Yorkshire hospital before being cast in this film. He won his role ten seconds after meeting George Lucas for the first time; all Mayhew had to do was to stand up.
Trivia: When the film was first released in movie theatres in 1977, there was no 'Episode IV: A New Hope' on the opening crawl, because the producers said that people would be completely confused by it as there were no other 'Star Wars' films at the time. It was only after the film became a success that George Lucas was able to put the words back on.
Trivia: Darth Vader appears for a mere 12 minutes in the entire film.
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.