[Fyron]

Not all multi-classing combinations are very viable... some work great, others do not. Fighter/mage can work as long as you are not trying to be an evocation mage. If you just want to do lots of damage from spells, don't take any fighter levels, stick with pure wizard or sorcerer. But if you want to use magic to enhance your fighting abilities (bonuses to armor class, abilities, etc.), it works out fairly well and you don't need more than 4th or 5th level spells for this. This also frees up the party's real wizard from worrying about using spell slots to beef up your character, allowing him/her to concentrate on other spells, such as more fireballs.