Greater Spell Focus( Player's Handbook v.3.5, p. 94)
Choose a school of magic to which you already have applied the Spell Focus feat. Your spells of that school are now even more potent than before.
Spell Focus (PH) ,
Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select. This bonus stacks 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 to which you already have applied the Spell Focus feat.