you wont have a shot at Helton. I think some owners might consider Delgado, Thome, Sexson, or Giambi, but probably not. From my past experiences its hard to get a preseason trade done. Owners draft certain players because they really like them and that makes it hard to get any trades done. I would try now, but dont give up too much. When season starts and people realize they need pitching that puts you in a much better position to trade.
Another option would be Maddux and Millar for a Sweeney, Teixeira, Lee, or Ortiz(he has been on fire in the spring)