That's a pretty broad question! First let me say that I have no experience with Microsoft RMS, but I do have quite a bit of experience integrating Great Plains with BV.

First you will need to decide how sophisticated you want your integration to be. At the most basic level it sounds like you will need to send orders from the website into RMS and update the website's inventory from RMS. So, there is two-way communication. It's unlikely that you will be able to perform an inventory lookup in real-time from the website into your RMS system without negatively impacting site performance. Instead you will probably need to setup some kind of inventory update process that runs at an interval that you feel is reasonable. The actual communication between the two systems could be handled in a variety of ways but using a web service is probably the most common. You could also use SQL replication.