Broadband sharing on W2K Server

I would STRONGLY recommend you get a router, preferrably a true firewall, for this.

If this is a domain controller, you will be exposing your DNS (and worse) to the Internet. It is not secure, and it won't be long until this server is some hackers playground.

If you install ISA on this server, it will slow down the server and limit it's ability to do both effectively.

Get yourself a firewall, and save a lot of long term grief.

yes. ahem.
if you do not have nat now you must be telling us that your server is sitting on a static ip bridged to you by your isp. hot hot hot. that is somewhay unusual maybe. other wise you probably getting a static address assigned via your login by your isp. you probably interfacing via standard broadband modem/router and probably is already doing nat for you and you mean something else, but what do i know? nothin...
i would add to zaf's remarks: harden your os also, per instructions found at www.cert.org and consider a dmz, at least, for your internet public machines...be ready to recover them when hacked, no matter what you do as public machines are well, public and no help for it. with security you can only improve your odds imho.
you sure it is NAT you wish to implement? because you wish to share internet connection and you wish the new server to be the 'gateway' for internet access for your network? NAT means stuff is getting routed. (with firewall examining stuff - good idea). you can set your server up to *be* a router and do nat but maybe easier and more secure for now to let standalone cheap(er) router do it. you almost for sure will have to port forward smtp ports and whatever else you need, such as ftp, streaming video, pcAnywhere...
if you get a router you could also set it up to be dhcp server. then just configure clients to be dhcp clients. they will get ip, subnet, gateway and all automatically.
if we are misunderstanding you why not visit support.microsoft.com and search under your os for howto NAT and come back with questions again?