| The confusion here seems to stem from what the difference between Magic Resistance and Magic Damage Resistance is.
Magic Resistance is a probability. It represents the chance a character has to completely "avoid" or "block out" the effects of a spell. A magic resistance of 10% means that there is a 1 in 10 chance that a spell will be bounce harmlessly off the character. The rest of the time, the spell works just like normal. Magic Resistance is a very powerful thing -- at high levels of Magic Resistance (100%), you essentially become immune to all magic (with the exception of a very small handful of spells that bypass magic resistance, such as Imprisonment).
Magic Damage Resistance is different. It represents a percentage of damage that gets "absorbed." With a 50% Magic Damage Resistance, a spell like Abi-Dalzim's which may have done, say, 60 damage will now only do 30. However, it is important to remember that spells like Fireball, Flamearrow, Cone of Cold, Chain Lightning, etc. etc. do not deal "magic damage", they deal "elemental damage."
So to answer your original question: having a Magic Resistance of greater than 100% shouldn't (and doesn't) do anything -- 100% already means that you block out spells 100% of the time, there is no such thing as a probability greater than that.
However, having a Magic Damage Resistance greater than 100% can produce interesting side effects -- you can actually be healed by magic damage! So if you had a Magic Damage Resistance of 150%, a magic missle or a skull trap would actually heal you!
A similar technique I occasionally use is to have Jahiera wield the Club of Detonation and wear lots of fire resistant gear (Red Dragon Scale, Helm of Defense, Ring of Fire Control). She gets her Fire Resistance up into the 150% range, and when the Club Explodes it actually HEALS her!! You gotta love it!  |