What are you looking to do with the photos you want scanned ?
If they are only going on the computer or in a digital photo frame, 300 dpi is fine. If you want to print them enlarged or view them as a slideshow on you big tv 600 dpi will be a much better option.
Scanned Photo Samples 1
Sample 1 - 1971 5x3" Black & White Photo scanned at 300 dpi and 600 dpi.
Click on each photo, to see the difference in size
Scanned Photo Samples 2
Sample 2 - Colour 1992 6x4" Photo scanned at 300 dpi and 600 dpi.
Click on each photo, then max zoom, to see the screen size difference
Scanned Photo Samples 3
Sample 3 - Colour 2002 6x4" Photo scanned at 300 dpi and 600 dpi.
Click on each photo, see the screen size difference of the same area sample