Well, as on the Thor thread, I'm going to disagree here. The problem is that very few huge bodybuilder guys can act worth a damn.
So long as the physique is in the right ball park, I think you'll get a better film with a competent actor than some physically more impressive plank of wood.
For example, I don't think Troy was a great film, but Brad Pitt made a good Achilles. He can't weigh more than a buck-seventy, but he's athletic, he moves well and he actually sounds like he might be the character. I like Arnold, but he was laughable in Conan. "...Und hear de lamentations ov de vimmin."
The Conan actor's physique looks OK to me; I'll reserve full judgement until I see it.