TEMA you don't have to convert a pattern from flat to circular needles. You use the circular needles to knit flat. Don't join the yarn, knit from one end to the other, stop turn the work and knit back.
Edit: eHow has information on converting flat to circular
Edit: Also do a search on "converting flat to circular" for a lot of information.
It is easier to do than to explain.
patterns for knitting sweaters in the round:
Look to the bottom of the page.