I've not read "God is not Great", so I can't speak to it. But the bible is held by most christians, certainly the fundamentalist "born again" branch to be the word of God. Period. They do not distinguish one book, one section or one verse as being more or less idiotic than another. That said, why wouldn't he go directly to it's weakest points? I would. There was a nice debate about circumcision on Hubski that was started via a Hitch post. I shall try and find it. Here it is. He is definitely not without hyperbole but he was also quite entertaining. Wish he were still around.
I wish he was still around too, for his turns of phrase. My point was, if you purport to be an intellectual atheist, you should be above trashing fundamentalists. It's too easy, and most "regular" Christians will agree with you anyway. So it cheapens your argument, and causes rational Christians to ignore you. The Bible is not held literally by very many Christians anymore, only those who happen to be most vocal, and arguing with people who think the world was created in six days is a supreme waste of time. EDIT: this basically came up in the discussion you linked, I guess, with the pastor calling his arguments "infantile." At least in God Is Not Great, I might agree to an extent.