hmmm... another loaded question from Juan... *G*

basically, you need a scanner with a tranparency attachment, or you could build one yourself with white plexiglass and a flourescent bulb... both of which yield mixed results, and often are of dudiously lacking in reproductive quality, especially when blowing them up to something usable like larger than 8inches. depending on what you're using it for, you might consider sending it out to a color house or service bureau for scanning and color correction.

the resolution issue is the same: ppi is determined by the final printing line screen and scale. say you are printing the image at 3 inches -- your slide is basically 1 inch. so if you're printing at 150 line screen, the scan would need to be at least 300ppi at 3 inches, or 900ppi at 1 inch.

The best way is to have a flatbed designed for scanning film, such as the HP Scanjet 4370 (low end) or HP Scanjet 7650 (high end). Failing that, you'll need a uniform light source to backlight the film (think of an invertable light table).

DPI will depend entirely on the size of the original and output size.

(BTW, if you're scanning a slide, you'll want to remove it from the little frame they come in, so it will lie flat on the glass. Emulsion side down is best.)