Fantasy help: Trade Larry Johnson for Plaxico Burress?

[FoxwoodsFiend]

We start 2 WRs and 2 RBs.
Right now my WRs: Steve Smith and Jimmy Smith
Running backs: Larry Johnson, Tiki Barber, Julius Jones, Reuben Droughns.
So basically-is it worth the upgrade from Jimmy Smith to Plaxico if I would have to start Julius Jones (who may or may not be healthy) or Reuben Droughns (who never finds the end zone)? Also, how long is Larry Johnson expected to play?