Strike me down, and I will become more powerful than you can possibly imagine.

Love Star Wars (at least, the original trilogy), but one of the things I don't care for is any hippy-dippy new age feelings-based stuff, "Stretch out with your feelings" Obi-Wan tells Luke. "How feel you?" Yoda asks young Skywalker in Phantom Menace.

Hokey religions and ancient weapons are no match for a good blaster at your side.

Pretty powerful, yes, since Anakin chose to join him after he died. Or did you forget that, Moonglum9?

he does became inmortal, which is a very nice power

I always figured that Obi-wan was talking about becoming a martyr. However, since George massively altered the original plans for Return of the Jedi it's possible that something else was originally planned for Obi-Wan after he was killed.

His transcendence allowed his voice to guide Luke. He could always be with Luke and speak to him at important moments. Therein lies the "power."

Telling Luke to run instead of staying put and having a shootout with Storm Troopers. Later telling Luke to "trust his feelings" when on the mission to blow up the Death Star. We can extrapolate from these moments to understand how the continued presence and mentorship would help Luke to become a Jedi.

But for Vader that IS a power that he can't imagine because it involves letting go. Being selfless.

Obi-Wan's death has the power to inspire wonder and hope in the force. That death is not an end in some scenarios.

Vader's idea of power is to destroy everything around that is a possible threat to him or won't bend to his will. That might is right.

Looking at the new Star Wars universe, there seems so many Jedis out there. By becoming a higher being, he could be basically contact all of them and unify, help them even when they were in disguise. Quite powerful.