Your signature shows that you have an 11-16. In that case, I'd be tempted to go for a 16-85 and a 70-300 for the super telephoto range.
this gives you essentially the same setup as I have
The 16-85 is a great walkaround, the 70-300 (the VR version, please) is a great telephoto zoom and, coupled with your 11-16 gives you a 16-450 range (equiv), and that's great.
Keep in mind, though, that this combination *will* be more expensive than the 18-200 alone. I bought my 18-200 for $800 and sold it for $700 a year later: I was able to get the 70-300 with that and put the rest towards a 10-24, which I've since bought.
I do think, though, that the image quality jump is more than worth it.