[Ygorl]

Only lightning resistance helps against lightning. The same applies to physical damage spells (spells whose effect is measured in hit points) in general - whether it's lightning, fire, poison, cold, big rocks, acid, whatever... If there's a resistance to the mode of damage-inflicting, it applies, and unless the damage is armor-piercing or armor-negating armor helps also, but magic resistance is not relevant. The exceptions are things like Mind Burn, Astral Fires, or Shadow Bolt, spells that operate on a less physical level, and those spells are noted "MR Negates".