Maggie smith does seem the obvious choice. She's just perfect really. Sharp, witty but with a twinkle in her eye.
Nanny ogg I think is the one that really has to be right, for me at any rate.
The reason I think Marie anne duff would be so spot on for Magrat is because she has that pretty but washed out look which I think Magrat has.
This is the trouble though really isn't it. It's really hard for an adaptation to please everybody and dodgy castings could be a make or break for people who already have an idea of what the characters look like