some do. some don't.
Unless you're using Ghost as a replacement for "Creator of the Universe" it does not conflict with atheism.
it seems like a dumb question, but when you think about it....
no, seriously.
Atheists believe that there's no God.
No God = No point in believing in an afterlife, either.
No afterlife = no point believing in "souls"
no soul or afterlife = no ghost hanging around cause it couldn't go wherever and didn't come back for it.
Agnostics, on the other hand.....