Re: two computers, 1 hd external
I just turned an old build into a home server, works great and was cheap. Originally I just had the server plug into my 1.5TB external drive with USB and share it on the network, but I bought another 1.5TB and put it inside the server, I use the external as a backup/portable server (since they have identical contents and I synchronize them often, I can take all the contents of my server by taking the external drive with me). With a real PC for a server you can also run media streaming/sharing, camera, printer share, etc.
My server is an AMD Sempron 2600+, 2GB DDR RAM, XP Pro with 120GB IDE main drive + 1.5TB SATA storage drive, not an exceptional build but it's plenty for a home server.