I need to take 500 screenshots at 325x200 over and over. How can I do this on a Mac quickly? I could just do Cmd-Shift-4 and draw them out, but that would take forever and kill my hand doing that 500 times.

Is there a screenshot app/utility out there for Mac that would allow me to set a specific size of a fixed box and just allow me to quickly take screenshots over and over just moving the box around on the screen?
If you know javascript you could use something like phantom.js feed it a list of urls to get screen captures

http://phantomjs.org/screen-capture.html