Unfortunately the 1.5 repro factor makes things tough. I love shooting people. My main concern is, and always has been limiting depth of field for subject isolation. The rule of thumb for portrait photography: Use a lense 2X the mormal focal length for head and shoulder portraits. The normal for 35mm photography would be 100mm since a 50 is considered the normal. with the 1.5 factor of the D70 you'd probably want to use a minimum focal length of 70mm. This translates to a 105mm. Understand this, distortion will occure with shorter lenses as you get closer to the subject. There is a distance of 6-8 inches from the nose to the ear. This distance will become exagerated when using to short of a lense. Your 18-70mm lenses has the 70mm(105) you are looking for. I own this lens myself and am very happy with it's ability.
I've discovered that most digital images, in fact all my images aren't what they should be until I've sharpened them, etc. in Photoshop.
My favorite lens for people, etc is the 80-200.
Best wishes for success.
The best lens is the lens that represents the effect you invisioned. I like shallow deep of field for subject isolation. Therefore a majority of my work is performed with long lenses at wide open apertures.