Shada,
I believe you are right on as for how "bane" damage works (vs. undead,
vs. grimlings, etc.). Verant has also commented that we should see more
and more of these as time goes on.
vs resist damage, on the other hand, is treated differently. The bonus
value is calculated as an increased DMG stat on the weapon, except that the
elemental portion is run through the resistance system before calculating
the strength of your hit. I believe the way that this would work is if you
had a 10/30 +4 fire sword, it would be treated like a 14/30, but the 4 (and
associated bonuses) would be given a chance to resist, so, depending on
resistance checks, the weapon would do anywhere from 10-14 / 30.

Talies