Greater Spell Focus( Tome and Blood: A Guidebook to Wizards and Sorcerers, p. 40)
Greater Spell Focus [General] Choose a school of magic to which you already have applied the Spell Focus feat. Your magic spells of that school are now even more potent than before.
Spell Focus (PH) ,
Add +4 to the DC for all saving throws against spells from the school of magic you select to focus on. This supersedes (does not stack with) the bonus from Spell Focus.
You can gain this feat multiple times. It's effects do not stack. Each time you take the feat, it applies to a new school of magic.