alexs,
Setting up an SME Server behind a router can become complicated. You need to decide what services the router, Win2000 Server and SME Server are going to provide and then set-up each one accordingly. Typically it is easier to just eliminate the router, if possible. If you are going to continue to use the router as an internet gateway/firewall for the network you need to provide the following information as a bare minimum just to get started.
Do you have a fixed or dynamic IP address?
Does the router or Win2000 Server provide DHCP service for the network?
How many NICs do you have installed in the SME Server, 1 or 2?
What services does the Win2000 server provide? Do you plan to move any of these to the SME Server?
What services do you want the SME Server to provide?
What is the network topography, peer to peer or Windows Domain?