magic weapons in my campaign setting

 A +1 bonus on a d20 isn't that big of an effect. For that reason, I consider mundane weapons of high quality, like "Masterwork" or "Exotic" weapons, to grant a +1 bonus to THAC0 and damage. I'm also okay with them striking monsters that are otherwise immune to mundane damage. 

Magic weapons glow with aetherial power and grant a +2 bonus to hit and damage. Some weapons only have magical power in special circumstances, such as when used against orcs or goblins, or when held by a character of the necessary alignment. 

"Double magic" weapons glow with even more brilliant power, and grant a +4 to hit and damage. 

"Triple magic" weapons are mythical in existence and in power. They grant +5 to hit and damage, and only unique world artifacts are ever that powerful.