Gilberto is a great player, but is he that big of an upgrade on Zanetti? I haven't seen him as much as some around here, but for Brazil, he looked like the same kind of player with a little more pace.
Priority 1 is defense, that much is clear.
Priority 2 is midfield, to me the ideal signing is someone with the defensive skill of Zanetti, but with the offensive presence of mind of Pirlo. Essentially, a Gerrard type. I'm not sure Gilberto is that kind of player, but if he is, then by all means, sign him.