might be a good idea to run a parallel network since you want to get to know how to do NAT, etc through your linux machine (get to know ipchains, it can do all the NAT and firewallin' you need). You could put two nics in the new comp, and have one on one subnet, and the other on another, and just unplug the main nic when you want to test out your NAT and port forwarding settings. that way you could enjoy configuring dhcp etc.