Shun Cheung , May 08, 2008; 08:43 p.m.
First of all, the Nikon 18-200mm AF-S VR should be below $700 new, but since you already have the 70-300mm AF-S VR, I don't see why you also need a better 18-200. Would you consider selling both the 18-55 and 18-250(?) Tamron and get the new 16-85 AF-S VR? That will get you a bit wider and overlaps nicely with the 70-300.
Nikon's AF system is designed for lenses that are f5.6 or faster. That is why I am strongly against those Tamron 18-250mm lenses that are f6.3 on the long end.