I've been scanning at 1200 dpi. In many older, smaller photos, there is a surprising amount of detail that just sn't seen when the photo is small. When scanned in at 1200 dpi, you are able to increase photo size quite easily. If the detail is there, you will see it in the enlargement.
Should you decide to turn your photos into video family trees, the 1200 dpi photos work fantastically. If you use 300, many times you see pixelization in the photos