Multiple AD Sites connecting thru VPN

We have two sites, H.O., and warehouse connected using 128k leased line. We have 4 W2k Active Directory at the headoffice site, 2 in our warehouse. That makes it 6 Windows 2000 DC servers. Both sites have Internet DSL connections.

Here is my question. If we remove the 128k leased line, can we use the Routing and Remote Acces of Windows 2000 to connect to the H.O. (VPN)?? Right now the VPN is configured at the H.O. for authenticating Internet users that connects thru dial-up using a local ISP. We are using Mandrake Firewall (shorewall) at the warehouse side and Stonegate at the H.O. side.
What are the ports that I should allow to be able to connect H.O. and Warehouse using Windows 2000 ROuting and Remote Access(VPN).???