Roseanne would be perfect for Ursula!
I think Helena Bonham Carter would be an excellent Maleficent. I think she's very gifted actress (though admittedly, I don't like *all* of her movies) especially when it comes to being evil. She's positively perfect as Bellatrix in HP. Plus she's got those uber high cheekbones and large eyes. . . I think it could work.
Gerard Butler could work as Gaston. I don't think they'd would have a snowball's chance in you know where getting him to do it, but that's a whole 'nother thread.
Uma Thurman for the Evil Queen- I know she's blonde, but I think she could do the transformation into Old Hag pretty well. Oooh, or Jennifer Conelly.
I've got nothing for Jafar. He'd be tough. I think Johnny Depp could pull it off, but I don't think he would ever consider it.