Real Madrid haven't played a 4-3-3 in quite a while, but it would definitely be an option to consider with Nuri Sahin fit.
Ideally, Nuri Sahin and Xabi Alonso would occupy the two more forward center midfield positions, while a defensive midfielder (Sami Khedira, Lassana Diarra, or even Pepe) would occupy the defensive midfield positions. Mesut Ozil could also be used instead of Alonso.
This formation would be an excellent one for Real Madrid to utilize against Barcelona. It would place control of the offense almost completely in the hands of Sahin and Alonso, the latter of whom has expressed great excitement at the prospect of playing alongside the former.
It would also give the defensive midfielder the obligation of breaking up play to feed the ball back to Sahin and Xabi Alonso, to create play once more and link up with the wingers, who would like be Angel Di Maria or Kaka and Cristiano Ronaldo.
Of the current defensive midfielders at Real Madrid, the only one who is really capable of performing the duties required of the defensive midfielder in this formation is Pepe.
However, since the Clasico Champions League matches against Barcelona last year, Jose Mourinho has been extremely reluctant to give Pepe the freedom to roam the midfield, and has preferred to keep him in defense, where has been solid as well.
This formation has the potential to be a successful one, but it would require Mourinho to make some very bold moves and decisions.