Maiar in physical form

[Gurthang]

I don't think that Sauron could regain his lost physical forms. I seem to remember a passage that says at the fall of Numenor, he lost his shape and could no longer appear fair to the eyes of men. That seems to suggest that he could not make that form again. I know that this is found in the Silmarillion, but I don't remember the exact text, and I don't have a copy handy. If someone could find that passage, that would be great.

As to being less powerful, I think that it is somewhat coincedental. In one instance, he needed to take the form of a wolf, because that was the only creature that would defeat Huor(I think that's his name). Other times, I think he thought that fear was more important than being powerful, after all, he could easily defeat many people with only a fraction of his power. But I also agree that the ring took some of his spiritual power.