The spell Fox's Cunning gives a +4 Enhancement bonus to your Intelligence Attribute. This would increase your Intelligence modifier ( And intelligence related skill checks ) by +2.
Also keep in mind when starting to get into the stacking of ability bonuses that you do not try to stack multiple types of Enhancement bonuses together. The Enhancement bonus from Headband of Intellect does not stack with Fox's Cunning. However, the Enhancement bonus from Fox's Cunning would still stack with the Racial Bonus you get for being a Fire elf, the inherent bonus to intelligence from a wish spell would stack with repeat castings of the wish spell up to +5, or the +2 alchemical bonus from Mushroom powder.
A good rule of thumb is that only Dodge Bonuses and Circumstance bonuses stack with themselves. Everything else does not stack. So if you have a +2 to Natural armor as your current race choice equipping an Amulet of Natural Armor +1 will not increase your Armor Class.
Don't forget that only permanent bonuses to intelligence will increase the amount of skill points that you get retroactively. A permanent boost would be a racial bonus, or an inherent bonus. Temporary bonuses such as a Headband of intellect or Fox's Cunning will increase the bonus on skills with a primary intelligence attribute but will not grant retroactive skill points.
When the headband of intellect states that the bonus will become permanent after 24 hours has elapsed, after that amount of time has passed you will still not gain retroactive skill points based on the bonus of the headband. However skill points gained from that point on will be granted if you gain a level.
If you're looking for a real permanent bonus, the Tome of clear thought is going to be your best alternative. However, because you need a wish spell to create them, they are rather pricey.