"R" stands for "Refresh", meaning it's shipped with support for new Xeon 5600 series processors. If you're using a non-refresh board, you'll have to update BIOS to 45 or later before you can use a Xeon 5600 series processor.
One of the main reasons why the refresh boards were launched was because it was easier to identify which board comes with the bios version 45. All refresh boards will have bios 45 installed.
Here is a link which can answer some of your questions:
All the best,