1. It would certainly seem that Sauron became more powerful through the creation of the Ring; however, elsewhere it appears that each Ainu had a finite amount of innate power. This could perhaps be explained thus: Sauron put some of his innate power into the Ring, but the Ring also had some power by virtue of its craft alone. Thus Sauron + the Ring is greater in power than Sauron alone.
2. Melkor was originally the greatest of the Ainur; he gave up much of his innate power, though, to provide power to his minions. Eventually, he so dispersed his power that he himself was relatively weak; remember he was nearly defeated by Fingolfin. This is briefly mentioned in The Silmarillion, and is more fully expounded in Morgoth's Ring.
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