[lathspell]

Sauron was in a way dependent to the Ring. If It would be destroyed, Sauron would be destroyed as well. But it's not so that Sauron wouldn't win the War without the Ring in his possession.

In my point of view the Ring itself ain't more powerful than Sauron, but the Ring is again dependent to the one it serves. If that one is already one with great power, then the Ring would give him the power to be greater than Sauron (for example if Gandalf or Galadriel had taken the Ring). As for the weaker ones, the Ring couldn't do much with: only devour them and make them corrupt and invisible (Gollum, Frodo and Bilbo).

So I guess that the Ring itself had no power that was greater than Sauron himself, but he gave it's bearer powers according to the powers he already had. So it wasn't the Ring that was so powerful in itself, but he could make someone so powerful that he would overthrow Sauron and that's what made it so important for both sides.