To me, Smith and Brown are about even in terms of what they can offer. In his prime, P.J. was probably more valuable since he was a better defender than Smith. However, P.J. wasn't in his prime last year and Smith is no slouch himself on the defensive end. Smith, like Brown, can rebound, play defense, and can hit the outside shot from 15-18 feet out.
As for Marbury, he can't be much less useful than Cassell, and that's coming for someone who was a Cassell supporter. I think Cassell could've done more last year for the team, but he certainly didn't help all that much on the way to #17.
With that in mind, I think Marbury and Smith would be at least as good as Brown and Cassell. Let's hope Danny can pull it off.