Is Wii more powerful than Xbox?

Wii is admittedly not the monster technical jump from GameCube that system successors usually are, which often leads to this question.  There's no easy answer, as the hardwares each have different strengths.  However, I tend to look at things in this simple way:

If GameCube at its best and Xbox at its best were considered in the same league last generation, surely further increasing GameCube's speed by 50% and doubling its RAM would push it over the top in most situations.