Greater Spell Focus( Forgotten Realms Campaign Setting, p. 35)
Choose a school of magic to which you already have applied the Spell Focus feat. Your spells of that school are even more potent than normal.-
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 overlaps (does not stack with) the bonus from Spell Focus.
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school* of magic