Spell damage is a capped stat against players but against monsters there is no limit. Stacking as much spell damage increase as possible will only add to the damage output your spells are capable of. with 150+ SDI and a slayer on the spellbook, you be doing 250+ damage fireballs to monsters. SDI can be increased in several ways, including wearing certain items of magical armor or clothing that have an SDI bonus, casting certain spells that increase SDI, or imbuing certain items with magical powers that increase SDI.
