How much resolution is enough?

It depends. When you digitize your artwork, film or picture, you must know what the final application is going to be. This information will determine how much resolution will be necessary for an optimum result.
There are three factors you must consider:

a) Final output: What size prints are you going to make? Are you making slides or 4x5 transparencies? Is it for your website? Is it for your business card, postcard or stationery?

b) Printer resolution: Are you using an inkjet printer; an Iris; or an LED printer, which prints directly onto photographic papers and films.

c) Media where you are going to print: Are you printing on canvas, fine art papers, film, or not printing at all?