Networking

I have a netgear router set up with a PC and a laptop. The printer is hooked to the PC but I can print from the laptop without it being a network printer, as long as the PC is on. My question is this, I just picked up Windows Server 2003 and would like to play with setting up a domain instead of a workgroup. Do I need to purchase a third computer to set up as a server, and how much storage, RAM, etc. should it have?