For reference, the rules for a lightsaber in Star Wars Saga Edition (which is very compatible with 3.5) are:

Medium-size exotic melee weapon (requires Weapon Proficiency: Lightsabers)
Damage: 2d8 energy and slashing
Weight: 1 kg
Special: Ignores DR and hardness

Is there anything wrong with that? It seems like a pretty good representation to me . . . more damage than a sword, but not overwhelmingly so. (For reference, a blaster rifle does 3d8, but doesn't allow you to add your Strength modifier to damage the way a lightsaber does.)