Question: How was Obi-Wan going to become more powerful after he was dead?
Answer: I agree with Darius Angel's comment. I also think Vader expected a certain sense of satisfaction after defeating his former master. In reality, though, he was still "owned" by the Emperor and was living with the consequences of his choices. Defeating Obi-wan did not change much for him. Obi-wan, however, gained the benefits that Darius Angel mentioned.
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.