Why do you want to take macro shots? of what? and when? Do you want to take a picture of an Ant's head? or an ant, or an ant on a leaf? If you want the first shot, then you'll need more than a lens - add on tripods with macro rails, special flashes etc. (plus the ant)
I don't think you'd be able to make that shot anywasy with a D300 because in lacks mirror lockup.
I looked at a few different solutions then went with a close focus adapter for my 300D.
The 50mm macro lenses have excellent optics and they're great at photographing flat objects like stamps.
A good lens (say a 50mm f1.8) plus an extension tube will get you reasonable magnification but the image won't be a nice across the entirety of the picture. That's okay if you're shooting a bug - they're not flat so the image will be fine. Your image sensor doesn't go full frame either so some of the distortion will be outside of the imaging area.
Extension tubes allow you to focus closer than the lens would otherwise.
John Shaw (who wrote an excellent book on the subject - the best I've seen 'Closeups in Nature), suggests that extension tubes should not be used on zoom lenses because focusing changes the composition and vice versa. He suggests getting a close focus diopter.
The knock on these is that at the edges of the frame there can be some distortion. But - the sensor in the 300D is smaller than a 35mm frame so much of this distortion will be outside the viewing area anyways.
The longer the lens you'll be able to take pictures from farther away. That is instead of one inch from the subject - you can shoot at 12 inches.
If you want to know more about the options then I highly reccomment John Shaws book. It's a bit dated but he goes over all the options, the pros and the cons to all of them.
Having a dedicated 100mm macro lens is great solution but it will cost a lot. Extension tubes are great but you have to carry them and you'll still need to buy a lens beyond your 18-55.
I went with a two optical element close focus adapter (500D), it's quite portable and when I'm in the woods I can easily add it to my everyday zoom for spur of the moment macro shots, then take it off for other work.
My latest photogallery (see the url) has some shots using the macro adapter.