[Elladan and Elrohir]

Gandalf would seem to imply otherwise in Book Three: "But Isengard cannot fight Mordor, unless Saruman first obtains the Ring."

Obviously, it's a matter of opinion as to how far that statement can be taken. It may merely mean that Saruman would never feel secure enough to challenge Sauron's might unless he first had the Ring.

EDIT: That "only Gandalf might be expected to master him" quote has been used a lot on the Downs to "prove" that only Gandalf could have mastered the Ring and thus Sauron. I will not do full battle with that argument, but will merely point out that Tolkien uses the curious phrase "might be expected," which implies that this is not a for-sure thing. I will also add that Saruman also fulfills the qualities of Gandalf that are mentioned in this passage: an emissary of the Powers, of the same kind as Sauron; and, as has already been mentioned in this thread, he was before Gandalf's fall and resurrection more powerful (or at least perceived to be more powerful) than the Grey Wizard.