Hi there,
it's perhaps a good idea to take a look at
Welcome to Photozone!. This website does good lens testing and avoids the all too common euphoria, whenever a new lens comes out.
I will not comment too much on the 18..200 lens. It offers a great range of course, but I srtrongly believe that it means compromises in other areas.
Keep in mind that the D300 is probably the most demanding camera for a lens, that you can buy. 12 Megapixels on a DX sensor!
My advise is this: keep it simple! Limit your lens purchases and explore first.
Cheers, Sean.