Right. Since the resistances don't stack, you get the benefit of the highest resistance, which in your case is 30/-. After the wizard casts a 40 point fireball at you, then, you character will take 10 points of fire damage, your spell having absorbed 30 points of it. Your spell will still be able to absorb up to ten more points of elemental damage though, but no more, so that means your character no longer has 30/- vs elements, but 10/-. Still, the game will use up your spell before using the item, so assuming another 40 point fireball is tossed at you, and you fail your reflex save, your character will take 30 points of damage, and your spell will have worn off, soaking up all the damage that it could. Now, finally, your item with damage resistance will kick in. A third fireball will have ten of its points absorbed. In short, with the item, you will always be able to absorb at least 10 points of fire damage. With the spell you can absorb more at one, at least initially, but the item and the spell do not stack, because damage resistance does not stack.
See [LORE]Damage Resistance[/LORE] and [LORE]Protection from Elements[/LORE]