I wouldn't sign another centre back until Hyypia retires. We aren't going to get a top quality player to come and be 3rd of 4th choice. I'd go with Carragher, Agger, Hyypia and then have the younger players compete for 4th choice. We've got enough centre backs in the reserves, Hobbs, Paletta, Rogue, Antwi etc so I'd let them compete and play them in the League Cup games and and dead rubber Champions League games.
If there is an injury crisis with our centre backs next season we can always cover it with Arbeloa or by playing 3 at the back.