A: For taking good pictures inside, a large aperture is necessary. A larger aperture lets more light in through the lens, which means that you can take pictures without having to resort to using flash. Also, a larger aperture gives you a shallow depth-of-focus, which means that you can have only one plane in focus and the rest of planes out of focus, which gives a really nice blurred background effect.
I recommend Tokina 50-135mm f/2.8 (see Amazon widget). This lens has a constant high aperture of 2.8, meaning that f/2.8 is available from 50 mm all the way up to 135 mm. By looking at relative sizes of different apertures, you can see the advantage of this lens over the kit-lens that comes with Nikon D80, 18-135mm f/3.5-5.6, where at 135 mm, the highest aperture available is 5.6.
