VPN 2 Sites

I have two sites currently:
Main Site - Windows Server 2003 and an SBS 2000 Server. The SBS server will
be replaced with a Windows/Exchange 2003 Server.
Remote Site - Windows Server 2003 w/ Exchange 2003.

All offices use Exchange for mail storage, basically. Each Microsoft
Outlook client is configured for POP3 e-mail, which is hosted by our ISP.

I want to connect the offices into a single tree. Right now, they are
separate domains, without a VPN or any type of connection. I have two VPN
routers which will handle the connection. Once this happens, is it possible
to get the two domains into a single forest, for the obvious advantages such
as management, address lists, etc?

Will I have to recreate a domain at the remote site, or not? Any other
tips/advice anybody can provide will be appreciated!

PM