1 Server as both PDC and Exchange Server

I have a quite capable NT SBS that is connected to a 10.0.0.x network with a NAT box doing its job to connect the LAN to the Internet via DSL.

What I want to do is set up Exchange to accept connections from any system offering mail addressed to the networks users.

I have been assigned a static IP, which is 206.xxx.xxx.xxx (withheld for security reasons), but when i put this into my server, the rest of the 10.x.x.x workstations can’t see it.

I know that the advanced TCP/IP properties allow for multiple IP addresses, but is that where I need plug in my 206.x.x.x(with what subnet mask??)? Or should I go with two NIC’s?

If two NIC’s, what should the subnet masks be?

Am I even thinking along the correct lines?