I checked with level 10 Magic Missile in a save at the end of D2:ED, and an Intelligence of 25 or 64 made no difference to damage.
In DKS Intelligence does help with spell damage. With a particular (named) undead opponent late in FoV, using level 10 Magic Missiles and Fireball, he took 113/695 damage, respectively, with an intelligence of 12, and 160/929 with an intelligence of 100. He appeared to be immune to Fire Wall (level 10).
