I've always pictured Liz Smith as Nanny Ogg, although she's too thin I guess. She's the right type in my mind though. She was the grandma in the Royle Family, and the old crazy woman in the first seasons of the Vicar of Dibley.
For Granny Weatherwax, I'd see Dame Judi Dench in that role. She's got the stare right. I still picture my own grandmother as Granny, more for looks than personality
Emma Chambers has already been mentioned for Magrat, she looks the part.