I have a gastly on Pokemon pearl. When I evolve him into Haunter, I'm going to transfer him into Pokemon black. Then, if I trade with another ds with Pokemon black, will he evolve into gengar? Thanks.
Yep, it will evolve into Gengar. I've evolved a bunch of my Pokémon by trading with myself before. (mainly from Black to Black 2, but it should still work if you trade between two copies of Black)
Source: Experience / Knowledge