If memory serves, when a character changes class and suffers the level drop, she can still cast any spell that she knows -- no matter how high the spell level (e.g. Recharge, a level 6 spell) -- and may also learn any spell for which she has a sufficient skill level in the appropriate school (Theology). Thus, a Faerie endlessly switching level from bard to thief and back again every two levels can learn Nuclear Blast once he reaches 98 Thaumaturgy, even though only a 3rd level character.
Casting spells, however, depends upon a combination of character level and spell level. Any known spell may be cast at a minimum level of 1. (So the 3rd level faerie bard may cast Nuclear Blast at level 1, and could still do so even if he changed class to a 1st level thief.)
The maximum level that a spell may be cast at is character level - spell level. Or something thereabouts. Play around with it a little bit, and you should be able to figure out the exact formula.