How to create a live preview on a website?

I’m building a web store for a friend who makes custom stationery (wedding invitations, birth announcements, etc.) She’d like to have customers choose the color, font and text, and be able to provide them with a preview of roughly what the invite will look like. It could be a live updating thing (like those spiffy fluther comment previews below), or it could be a button that you click to see the preview – either one would be fine.

I’ve never built something like this into a website before, so any hints on where to start would be appreciated!