It depends what your intentions are.  If you are going to be doing some print work you'll want to scan at least at 300 dpi.  300 dpi is standard print resolution.  If you are going to be making print enlargements and have the space available on your hard drive, I would scan at 720.

It's safer to scan higher (if you've got the space) if you know that you may use your photos for a multitude of formats.  The rule with resolution is: you can always go down, you can never go up.

If you know that you are just going to be using photographs for web, you can quickly scan them in at 72 or 100 dpi.  72 dpi is screen resolution: you do not need any higher resolution for displaying on the web (because then you have to shrink down the image size to fit and you're just taking up extra space).  You can always export a file that is open by File: Save As and saving a copy of the open image for instance as a .jpeg (the most favored format).

When you Save As: you are creating a copy of the current state of the document you have open and depending on the format you choose, you are decreasing it's file size.  I will usually save from .psd to a .jpg on File: Save For Web within Photoshop (I still haven't gotten very deep into Image Ready).  This will allow you to preview how it will actually look at the file size you choose (you can adjust the quality).

It took me a while to understand that I don't want 720 resolution for the internet (yes I admit it but I learned most everything myself-please don't do that), I only need 72 resolution.  So if I have a master Photoshop file (usually I'll have 2 of every image) as a .psd which I can use for enlargements, designs, print that I can easily Save As (a .jpg) for the internet use.  If the image is too big simply go to Image: Image Size and bring down the resolution to 300 (print) or 72 (web) or change the image dimensions itself if necessary.  Note: Constrain proportions in the Image Size box will simply keep your image scaled properly.

Now you should understand a little bit more about resolution!  Read the full article on resolution.

- Article by Orion Williams copyright 2004