Resizing photos from a directory

Most need to be resized to 800x600, while others need to be resized to 640x480

What is the characteristic based on which the distinction is made? Is it the name, size, … or something else that can be assessed automatically or will you have to sort the images manually?

Scripting is more powerful then actions however I do not think one is any faster then the other. Its more a case of how many Photoshop steps are performed. In your resezing case it matters less for the action you use will most likely use Photoshop Plug-in "Fit Image" which since CS3 is a Photoshop Script.   All that is needed are two  single step action. that records a step menu File>Automate>Fit Image and in the fit image dialog you set 800 x 800 and the second one you set 640x640 your 4:3 aspect ratios images will be resized to 800x600|600x800 and 640x480|480x640.  These action would then be batched using menu file>automate>batch.