Thomas,
You may be able to pick up some close up filter lenses that work with various lenses. I have the Nikkor 70-300mm AF-D ED lens and picked up a Nikon 5T and 6T lens. These are actually filter rings with a diopter lens installed that allows my 62mm threaded lens to work as a macro. You can stack them to get additional magnification, which is an advantage of purchasing a couple of them. There are other manufacturers out there and Nikon also makes them for 52mm filter threads. They (the Nikon ones) are only $30 (52mm) or $46 (62mm) each, which is a lot less than a dedicated macro lens. Give them a try to see if you like macro work. If you want to get more into it, then you can invest into a true macro lens. Otherwise, you could be spending a ton of money for a lens that you may use only sporadically. This way, if you don't find macro work to be something you'd like to do a lot of, then you are out less than $100 and still have them on the off chance you need it. Be aware that these close up lenses don't match the quality of a true macro lens, but they do a very good job. The main advantage of them is you can use them on your existing lens and not have to lug a macro lens around in the gear bag.
If you want to get into macro work, a dedicated macro lens is definately the way to go. The 60mm would be a 90mm equivalent with the D100 and the 105mm would be158mm equavalent. The dedicated macro lens can also be used as a prime lens too, so it does serve more than one purpose. A longer focal length allows you to stay further away from skittish subjects (bugs or small animals), while the shorter lens would allow you to move in closer. You need to decide what you will like to shoot (flowers vs. animals) and make your purchase appropriately.
Enjoy, Dave