Easter egg: When you insert any of the 6 movie DVDs in the Star Wars saga, once the FBI screen pops up (red in Episodes I, II, IV, V, VI; and blue in Episode III), enter a number between 1 and 3 (and press Enter if needed). Once the Star Wars logo and the Episode number disappears, a menu with the planet that you chose (and not what the DVD randomly chooses) will load.
Easter egg: In all the Star Wars movie discs, and the bonus materials discs with them, when the red screen with the big word "Attention" appears after the FBI screen, you can actually disable all the menu transitions by pressing 1 (and if needed, Enter) when it comes up.
Easter egg: On the 4th Bonus disc of the Star Wars Trilogy DVD set select the "VIDEO GAMES AND STILL GALLERIES" option. Type in 11 ENTER, 3 ENTER, 8 ENTER. A yellow box will appear behind R2-D2, Select it to watch DVD credits with Bloopers/Outtakes.
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.