The quote from the movie (uttered by Ben Kenobi to Darth Vader during their duel on the Death Star in Episode IV) is:
"If you strike me down, I shall become more powerful than you can possibly imagine."
When I first saw the movie and heard this line, I was seven years old. I didn't really comprehend what Obi-Wan meant by this. He becomes a disembodied voice throughout the rest of the movie, guiding Luke at some critical moments. I remember thinking that was cool enough, but not "more powerful than you can possibly imagine.".
By the time the next movie rolled around, I was a little older (ten) and had some time to ponder what Kenobi had said. Also, in The Empire Strikes Back, we see that Ben is not just a voice, but now a ghostly image that appears to talk to Luke (and Yoda). Again, I remember thinking that this was cool, but especially 'powerful'. I kept expecting there to be more to it. Like perhaps Obi-Wan would suddenly appear to help Luke at some point in a physical manner.
By the time Return of the Jedi came out, I had given up on seeing a dramatic display of 'power' from Obi-Wan. I came to understand that he was speaking metaphorically, about his ability to help and guide Luke from beyond the grave. This WAS the power he was talking about, the power to influence Luke in a way that Vader could NEVER match or stop.
Lately, however, I have come to see an extra layer of complexity in this- one which I doubt was intended. Even so, I find it interesting. I have spoken before about how, when you take a step back and look at it, Obi-Wan and Yoda seem to be manipulating Luke into a confrontation with Vader. They purposely withhold vital information from Luke (like the fact Vader is his father) likely because they do not trust Luke to do the 'right thing' when he meets Vader. It seems (to me at least) that Yoda and Obi-Wan WANTED Luke to kill Vader. At the very least, they seemed concerned that if Luke found out about his true origins he might 'choose poorly' as his father had and perhaps even join him.
If you accept that this kind of manipulation was going on, then it is possible to look at Kenobi's statement in another light, namely, that of martyrdom. Obi-Wan knew that if Luke saw Vader strike him down that he would be inclined to hate the Sith Lord for it. Ben had, in a short time, become a father-figure to Luke. To lose that relationship in such a brutal manner (seeing Obi-Wan struck down while seemingly defenseless) would have a huge psychological impact on Luke. And it evidently did. For three years at least Luke was determined to face and kill Vader for what he had done. Indeed, upon meeting him on Cloud City, it was Luke who acted aggressively first- ready to do battle to the death without preamble. When Luke is told the truth of the situation by Vader- that he is indeed his father, Luke immediately rejects the thought because he has been conditioned by the lies Ben told him- lies which were given much more weight by Obi-Wan's death at Vader's hands. In short (too late), part of the 'power' Kenobi was talking about was the psychological impact his death would have over Luke.
As always, I'm sure I'm reading more into it than the writer(s) intended, but I do think it is plausible, and interesting.