If you want it to view on your PC screen or to post on the www, the higher the resolution, the larger the image will be displayed. 100 dpi will give you approximately a full chip size display. 200 dpi will be about double size, etc. I usually use 150 dpi for TheChipBoard posts.

If you are going to print the image, you can generally set a ink-jet printer to print at different resolutions. If your printer can print at 600 dpi (as some can) you would want to scan at 600 dpi to get the best print. To see the difference in these higher resolutions, you need to use a photo-quality paper, probably one with a glossy finish.

If you want to use the image on eBay, and you want the the buyer to be able to use the "enlarge" function, you should scan at several multiples of 100 dpi. This will give the viewer a larger image when he asks for an enlarged view. eBay reduces the resolution of your hi-res scan so the normal picture in the auction will fit. Lots of sellers don't know about this and the "enlarge" doesn't enlarge at all.