It depends who you want to trade. Looking at your team, I would try and trade Lopez, Sexson, Wells, and Baldelli. These guys would rank in value as:
For Wells you could rope in a top starter (Santana aside). I would target Peavy or Halladay. For Sexson, Lopez and Baldelli, you could round up a second tier starter like Kazmir, Haren, or even some hyped-sleepers such as Hill.