It depends. Not enough resolution, and you image will look "blocky" or pixelated. Too much resolution, and your computer will take longer than necessary to process the information but the printed result will not be ANY better.
The ages-old rule of thumb is that images (graphics) should have a resolution of twice the lpi (lines per inch) of the printer you will be using. Most desktop inkjets and such print at about 85-100 lpi. so if you are scanning an image and placing it in your publication at the same size, anything over 200 dpi is a waste of pixels. By the same token, if you're scanning a large image at placing it at 50 percent of its size, more than 100 dpi is not necessary.
Conversely, if you have a small image that has to be placed at twice its size, you would need 400 dpi with the above printer. If your work will be hi-res output from a commercial printer who prints at 175 lpi, you would need 700 dpi images (175 x 2 x 2). A good source is ScanTips.com http://www.scantips.com .