Oh, I'm so very happy to hear about Catherine Tate and Doctor Who! I'm really smiling over this.
I am sure there are those who will grumble and disagree, but having seen her both in the series and in the Comic Relief skit...these two actors are just...brilliant. They have a wonderful on-screen chemistry. In a vague way, they do sort of remind me of Bogart and Hepburn....I mean, in the way the just seem to bounce off of each other, interacting..weaving the performance of two individuals, into a beautiful tapestry that just melds them together...oh, that's a bit drippy, isn't it? Well, I do love watching them. Like a verbal tennis match, when they get into their stride...what fun!
GilraenH
Pro 
You're absolutely right about the chemistry. Donna will truly baffle the Doctor and vice versa. Which is what the old man needs! Can't wait.