1200 is a good number. Bearing in mind that most home printers print at 300 ppi, a 1"X1.5" negative at 1200 can be resized to 4"X6" at 300 with zero quality loss.
Check out the new podcast that M0g mentioned on the front page -
http://photoshop.weblogsinc.com/2006...odcast-debuts/ - to learn a little more about resolution.