It all depend how you are printing.  For use with a high resolution inkjet  (2880x1440 ink droplets per inch) via Corel's photopaint, I can achive profesional quality below 300 dpi. For commercial (4 colour offset via photolithographic plates) 300dpi is a good compromise between the lines per inch of the screens used to create the four halftoned images necessary to create the dot-on-dot colour. Also when laying out a commercial job, the image size is determined by the area (usually in inches or cm) of the image within the layout program.  300 dpi has enough dots so that the set of the evrlaid dots does not blur the image unaccceptably.

I used to do graphic arts, darkroom and presswork for a commerical job printing business through highschool, university and through the following year while I hunted for an engineering position.

Pass no temptation lightly by, for one never knows when it may pass again!