In addition to the article What does a RIP do? Global Graphics has published an article on calculating the size of the required data rate of a RIP. This is the first part of a two part series on Choosing the class of your Raster Image Processor (RIP).
The article links to an Excel spreadsheet with several calculation formulas for sheetfed and web printing. The formula for sheet-fed printing can also be used to calculate the amount of data for a platesetter.