I recently bought IWD, and was wondering about how it relates to the xp limits.

The question presented itself to me when I was looking at the class progression tables, and I noticed that while the xp limit is (as I recall) 8 million, a mage only needs 7.5 million to hit level 30. Does this mean that I can use 500 thousand xp to get a fighter to level 10, and then dual him to a wizard, and still level him up to 30? Or will it also limit the level to 30 total?

There is no XP limit in the game, only a level cap which you won't reach when you play the game the normal way.
Dualclassed cleric29/mage30 or multiclass fighter30/mage30/thief30 are possible when soloing HoF mode or using other ways to increase XP gain.