On Oblivion
Your base Magicka is determined by multiplying your Intelligence by 2, then adding in any Racial and Birthsign bonuses.
No Birthsigns give Intelligence boosts.
Increasing your Intelligence past 100 will continue to give you increased Magicka.
 On Morrowind
Affects your total Magicka and magic use. Governs the skills of Alchemy, Enchant, Conjuration, and Security.