The 18-70mm will do f3.5 at 18mm and f4.5 at 70mm.
I don't agree with what was said about the 105mm and the requirement for a full frame camera. The D300 has a 1.5x CROP FACTOR not magnification. The image is in no way magnified, but rather simply cropped. Therefore you don't actually loose image quality, you're just not using the far edges of the lens. In addition DX lenses take this into consideration and when used with a DX body you will use the entire frame from the lens, the FX bodies can still use these lenses but don't gain anything from them.
Full Frame
1.3x Crop Factor
1.5x Crop Factor
1.6x Crop Factor
The D3 and D700 are fantastic bodies and have large advantages but don't think you need to spend that kind of money just to use a macro lens. The 105 will work very well with the D300 and other DX bodies. Also keep in mind if you buy an FX body, don't waste your money on DX lenses, you have to buy all the expensive FX lenses if you want to take advantage of the full frame.