dpi = dots per inch. It's basically how high quality the image can become. (ie, if you were to zoom in on a 100 dpi image, it would be nothing but little squares and blur, but on a 600-1200 one, it would be highly detailed, if the source image itself is detailed). Unless you plan on photo manipulation and have the printer that can actually handle 4800 dpi, I wouldn't worry about it. Although most new scanners can handle that much...can't they.... I can't remember, haven't looked in a while...
If you plan on doing CG pics, Photoshopping and the like, here's a ref: for all of mine, I seldom go above 100, and if I do, it's usually 150dpi. That's because otherwise the image gets way too huge and takes up way too much drive space (at least on mine, I have no idea how much drive space you have...)