Buffon would have an incredible impact on the team.
One of the most important traits for a GK to have is a commanding, confident presence. Say what you will about Dida, but he has NEVER had this. Want to know why the team has always had problems with setpieces? Dida. The best defenses always have someone between the sticks that is assertive---he yells out who to mark who and, if need be, makes a decisive intervention. 99 percent of the time, Dida stands in the box with no clue looking like someone's holding a gun to his forehead. His timid-nervous demeanor effects the whole defense in an extremely negative way.
Buffon is the gold-standard for GKs. With Gigi behind them, (no homo

) Nesta and Kaladze/Maldini/Gallas/whoever will be infinetely better.
Buffon in--Dida out. Next step, midfield.