: : Gandalf enlisted the dwarves to reclaim their treasure but he obviously knew that they could'nt get at it without killing Smaug, and once a dwarf sees gold that he thinks is rightfully his he won't stop until he gets it. Gandalf also mentions that Smaug could have been used by Sauron for evil purposes, my guess is that with his foresight Gandalf knew that the Dragon would be destroyed somehow if the dwarves returned to the Lonely Mountain.
: If Smaug was still alive during the war of the Rings things could have been much worse in the north, as for the ring Gandalf did'nt know, it did seem though that might have guessed about the ring because of Bilbo's invisibility, (he knew noone could surprise Gimli that well) but he probably thought it was one of the seven because he spent years studying to find out it was really the one.

Cyberlock has it right: Gandalf was principally interested in eliminating Smaug as a potential ally for Sauron. Sauron might have used the dragon to lay waste to most of the North.

Gandalf might add, however, that he was "meant" to help Thorin; that he was "meant" to acquire the key and map to Erebor from dying Thrain in the dungeons of Dol Guldur; that he was "meant" to have a chance encounter with Thorin in Bree; that he was "meant" to enlist Bilbo in the quest of Erebor; and that Bilbo was "meant" to find the ring. The obvious strategy of destroying Smaug might have been impelled by greater forces in play.

But there's no need to take our word for it. I recommend the chapter "The Quest of Erebor" in UNFINISHED TALES -- if you don't have it, it's a must-have for any Tolkien fan. There are lengthy previously unpublished excerpts by Tolkien -- in the form of narrative explanations by Gandalf to Frodo & the Companions in Minas Tirith after the conclusion of the War -- which give some considerable treatment of the background of the events leading up to THE HOBBIT.

Enjoy...

Oberon