Beware lenses with a maximum aperture of f6.3 at the narrow end. If you really want an 18-200 then get the Nikkor though it is a pain to carry. Personally I prefer the 16-85, although I own a Nikkor 18-200 (which I must get serviced by Nikon to correct front focus and loose zoom action). I understand that the 18-55 Nikkor VR kit lens is also highly thought of, and is way above the quality of most brand's kit lenses. Nikon's VR is very good.
Don't get me wrong. There's nothing wrong with Sigma. If you had asked about DX size wide angles then I would have been suggesting Sigma over Nikkor unless you were suggesting that budget was totally irrelevant.

Body only plus lens is more expensive than body plus lens kit.