Fast focusing is a function of the lens (at least, with the Canon line. I'm guessing that's true of Nikkor lenses too). Buy a professional lens and you'll get fast focus.
You are already familiar with Nikon so I'd recommend you stick with that (especially if you already have some lenses). But if you are going pro then consider getting a full-frame body (such as Nikon D700), which will suit you much better.


