Hi xanda, I can only echo what the others have already said about the 18-200mm, I have spoken to a couple of people who use this on a D80 and they are very pleased with it as it covers a huge focal range and the VR gives you the equivalent of a couple of extra stops for low light situations.
For true macro (1:1) you will need a dedicated macro lens, the Sigma as mentioned is very good but you might to want to also look at the Nikon 105mm VR as it can double up as a portrait lens (plus you have VR).
Either option is expensive so you might want to have a look at a close-up diopter (which could go on the front of the 18-200mm) or extension tubes which will give you an introduction to macro without breaking the bank. I wrote a quick article about this a few months ago:
A beginners guide to macro
Hope this helps.