Id consider a tele-zoom that compliments your current lens rather than replaces it. A 55-200 f3.5-5.6 lens should be pretty inexpensive. A 18-200 will be considerably more spendy and do nothing to improve the quality of your glass. Or if you're willing to spend more look for something like 55-200ish range with a constant f4 or f2.8 (Though you're talking $200 vs $600 min most likely.) Im sure the nikon guys can give you more specific suggestions.

^ I can mention more specific lenses, but he's pretty much got it covered. I don't know that you care that much, but the 18-200 makes some compromises in image quality to have a such a large zoom range. It's a good allaround lens, but I'd recommend the 55-200 VR (~$200, I think), or one of the various 70-300s Nikon has floating around. I had my 70-300 4.5-5.6 VR for sale, and it still might be if you're looking for something like that....

I highly recommend the 80-200 2.8, you can pick up a used 2 ring version for around $600. I bought mine used over 3 years ago for $600 and sold it last month for $575. I probably could have gotten $600 for it if I was not in such a rush to unload it. Now that I think about it though I do not think that lens has it's own focus motor so I am not sure it will work for your camera. That would really sux.

I think there is an internal focus version, the AF-S but those run about $900 I believe and are a little harder to find.

You don't mention a budget so I have to throw in the 70-200 2.8 VR, I am pretty sure that is internal focus so you would be good. It is a classic lens. New they run about $1600 but worth every penny.