Is buying a 70-300mm lens for my nikon d40 better than buying a 3.5x telephoto lens?

I have a nikon d40 with a 55-200mm lens. Im wondering if I should buy the 70-300mm lens or just buy a 3.5x telephoto lens to add to the lens i already have. Will the zoom go as far as the 70-300mm, and will the focus still make the background blurry if i go with the 3.5x lens? Or will it be best to get the 70-300mm?