I simply have a hard time coming up with all that many female actors who are genuinely funny and none approaching Murray or Aykroid--though, obviously, almost no men are near that level, either.
What I fear is that they are going to change the concept. Ghostbusters is about a group of intelligent semi-losers who bumble around in a self-depreciating way for most of the film. That the main characters are partial screw-ups who have to struggle to find the resolve to be true heroes is what makes the movie great. Hollywood doesn't really like to portray women that way, however, so I could easily see this being turned into a statement about brilliant women who are fighting against the male establishment, rather than against their own failures. If so, that would make this something other than Ghostbusters. I'd also be surprised if they allowed a female to play a character as goofy as Louis (Rick Moranis).
Those two things--the possibility that the cast won't be that funny and that they'll try to turn it into a feminist statement film--don't leave me very optimistic.
We shall see.