I've read all 3 books. Fellowship twice, thank you very much. And I've seen the movie 10 times now (9 of which in the theater, once now that I've managed to find 3 spare hours in my schedule to watch my DVD). I am a sponge for useless facts.
But no one has, so far, been able to explain why Sauron needs the one Ring.
He's already got forces in his favor. He's already taking over the world without it. He's powerful. He's great, and mighty.
The best explaination I've gotten is that he wants a body.
Which reminds me: How exactly does not having the Ring keep him from having a body, and how does destroying the Ring bring his downfall?
Because, expert as I am on all of the useless stuff, apparently everything important has slipped past me. And everyone who can only answer that he wants a new body.