Ginger should be played by Christina Hendricks or Isla Fisher. This is based mostly on looks, although Christina's not a bad actress. I haven't seen Isla in any roles that have displayed her acting skills, but let's be honest...it's a remake of Gilligan's Island. I haven't paid attention to who the director is, but I foresee this being a goofy remake (goofier than the original television show), similar to the recent Brady Bunch movie.
At any rate, Isla probably looks more like the original Ginger than Christina does, aside from the eye color. However, if they want to go the route of giving Ginger a lot more sex appeal, then they definitely should go with Christina. Both are hotter than Hades, but Christina has a porn star physique. I'd probably prefer Isla for the role.
As far as Mary Ann goes, there are a ton of brunette actresses you could throw in the mix. Jessica Alba may be a good choice if they're "updating" the look of the characters. She can pull off the bubbly personality yet still be hot as hell. Rachel Weisz is a possibility. Hell, they could even give Isla Fisher brown hair and she could pull off Mary Ann. Keira Knightley, Kate Beckinsale, Natalie Portman, Rachel McAdams...the list goes on and on. For a comedy, however, I don't think you can pass up Mila Kunis. She's not a comedic genius or anything, but she does moderately well in the genre, and she can definitely make the movie have additional sex appeal.