If you can get either Faulk or LT for Lewis straight up, I would do that. Faulk catches more passes than LT, but is also a much riskier guy to have. Shawn Alexander straight up for him too would be worth considering, but if you want consistency, he might not be the best bet.
If it comes down to getting Ahman Green, Barber, or Garner, they are all at about the same level as Lewis in most rankings I've seen. (Garner is lower, but you have to upgrade him in 1 point for reception leagues). If you are going to trade Lewis for one of those guys, I'd look for a second player thrown into the deal given Lewis's current sky high value, maybe a solid WR upgrade or the like.