Martinez has major league experience with the Royals, but also has a reputation as a bit of a headcase, and while he can make flashy plays, he often botches routine ones. He basically takes the spot that Augie Ojeda has had for the last couple of years, the infielder who might get a callup in case of injury.
Dubois, Jackson and Kelton are, of course, all prospects, and all of them, I think, will get a shot at replacing Moises Alou in 2005. What the Cubs ought to do is try them all in right field (Kelton actually played some RF in the games I've seen), since that's where a replacement ought to go. In 2005, Sammy Sosa ought to be moved to left. It probably won't happen, but it should.