CentOS 5 Networking :: Use Webmin To Set Up Port Forwarding/NAT On A Dual-NIC Centos 5.3 Box?

Can anyone here point me to a walk-through or discussion of how to use Webmin to set up port forwarding/NAT on a dual-NIC Centos 5.3 box? The layout will be simple:

Internet --- NIC1 [CentOS Box] NIC2 --- Switch to other PCs

We have a BUNCH of exposed services that are on special ports -- for example, to connect to one machine, you go in with [IP_Address]:12000, and to connect to another, [IP_Address]:12002, etc., etc. We're currently using OpenSuse 10.3 on this box, and YaST makes this criminally easy (you give it the incoming port number and the destination IP/port numbers and it just works). But OpenSuse 10.3 is nearing EOL, we're buying a new machine, and I'd like to use CentOS on the new one.

I've read the sparse Webmin documentation in their Wiki, and it leads one to believe that you simply insert a "NAT" rule. But there's obviously something they're leaving out. I *am* opening the ports in the firewall. But when I log in to [IP_Address]:port, it just times out. The port forwarding never occurs. The test in this case is SSH, and I know that SSHD is working properly because I can log into that machine just fine from another PC on the same internal subnet.