Resolution and color depth are the two most important scanner specifications. The optical resolution equates to the number of sensors in the scanner's scan head, and the more sensors the higher the resolution. High resolution translates into smoother curves and lines, so look for a scanner with a high optical resolution of 600 dpi or so. The interpolated resolution , though higher than the optical resolution, is the result of a software algorithm that adds dots to the optically scanned image. Though interpolation can double, triple or even quadruple the optical resolution, image quality isn't always improved.
Color depth refers to the number of colors that the scanner can electronically capture.
Some scanners, the Canon CanoScan N656U for example, capture 42-bit color or well over several thousand trillion colors. If you're a photo editor, you will probably appreciate this extra data. Don't forget that as you increase the color depth of your scans, you'll also increase the size of the resulting file. For each extra bit of color depth you use, you'll double the file size. SCSI offers the fastest data throughput, but unless it's already installed on your computer you may be looking at a real adventure.