I am posting this message in reply to the first disscusion regarding scan resolution.

Is there a difference between Dots Per Inch and Pixels Per Inch? I can't seem to find any reference to dpi in PhotoShop 5.5. Everything is ppi.
The way photoshop treats it is that ppi is always refered to in relation with print size. My understanding is that any change to ppi will only change your print size.
In other words your monitor resolution is already set to 640 x 480 and your image is 64 x 48 it will take up one tenth of the screen. The only variables to be changed are the image size and/or the screen resolution. Dpi and ppi will not effect it in relation to the monitor.