Happy new year (am I still allowed to say that?)

I would like to make a minor modification to the checkout area, but am having trouble finding the correct file(s) responsible.
When you got through the checkout steps (or the single page checkout), when you get to the end (and have Paypal configured) and click Checkout, I would like the checkout to open in a Modal Dialogue or New Window (with everything removed except the URL bar).
I know how to do that, but I cannot find which files are responsible for creating the button {button_xxx} etc.
Thank you very much for your time.
