I think I read that Ghost Touch weapons can crit vs incorporeal undead. If that is true, then would an incorporeal creature that fails its save vs a Ghostbane Dirge spell, which practically gives all magic weapons the Ghost Touch ability vs it, be critable with them?
Nope, the spell only does what it says it does. Ghostbane Weapons need *something* special all of their own, with all the ways you can damage incorporeal creatures.