you are going to need an additional NIC.
As I remember, you will want to have the incoming connections from the router on one, and if you want the server to be a gateway, you will need to keep that separate for the cable modem.
Since you are doing a small network, I would advise doing static ips so we don't have to get into subnet masks and the DHCP pool of addresses and all that jazz...
Cool - I think I've got a spare in one of the boxes I have yet to unpack - even more motivation to finish unpacking. I figured that's probably how it would go, but it's a matter of sorting out the details. I'm thinking it'll be fun just to play with.