Karinne,
Back in the pre-digital days when almost everyone used a 35mm film camera, a 50mm was and is considered the standard lens. It is called the standard lens because it records in the typical photograph, i.e. a figure in a landscape, pretty much the way we see the world. Any lens shorter than a 50mm lens, like a 35mm lens etc., is considered a wide angle lens. Any lens longer than a 50mm like a 100mm lens is considered a telephoto lens. The 50mm lens works well for a figure in a landscape when the landscape is the primary subject. When you get up close to the figure and fill the frame with either a 3/4 or a head and shoulders shot, there is significant distortion. Noses get longer. Hands get huge if they are on a different plane than the body. You have to go to a telephoto lens if you want to fill the frame with your subject and record him/her without distortion. Photographers have had arguments for generations over what is the best lens for doing this or in other words what is the best portrait lens. In the 35mm format some claim an 85mm is the best, to my eye there is still some center distortion. Others claim a 135mm is the best, I think at this length the face starts flattening a little too much. From my experience, in the 35mm format, most professional photographers would say that a lens somewhere around 100mm is the best portrait lens. That's the portrait lens I have on my full frame or 35mm format camera.
Now that we're in the digital age, that 100mm portrait lens has to be translated to the kind of camera you have, to the size of the sensor in your digital camera. Although professional digital cameras have sensors in them that are the same size as a 35mm frame, many of the prosumer cameras, like the D40 you wrote about have sensors in them that are smaller than a 35mm frame. When you shrink the size of the sensor you are in effect lengthening the lens. This is called the crop factor or focal length multiplier or field of view crop (different words for the same thing). Every camera is different. The Canon D40 has a crop factor of 1.6. The Nikon D50, the camera Laurel has, has a crop factor of 1.5. That means a 62mm lens on a Canon D40 (100mm divided by 1.6= 62.5mm) and a 66mm lens on Nikon D50 (100mm divided by 1.5= 66.66) are the equivalent of a 100mm lens on a 35mm full frame camera.
A really good website to find out all about photography is DPReview (
http://www.dpreview.com/), go into the discussion forums and you'll be able to acquaint yourself with every aspect of photography. There are professional as well as amateur photography questions being discussed. DPReview just did a review of the Canon D40. Check out fredmaranda.com also (
http://www.fredmiranda.com/reviews/) for reviews and comments about lenses.
If you're going to shoot photos indoors using natural light, you need a lens with a very wide aperture. Spend the extra money to get a lens that opens to f2.8. F2.8 is what photographers call a fast lens. Fast lenses are worth every penny.
Theorectically, a zoom is the perfect lens, but there's often a trade off in detail. Zooms sometimes are a little soft when compare to primes, or lenses of one focal length. Go to the camera store with a compact flash card and take photos with a zoom and a prime lens on the camera you want to buy, bring the card back to your computer and see if you notice a difference.
And finally, yes, you can use the same lens both both to photograph your subject and to photograph your art work.
Hope this helps. Steve