Building home server

I'm planning on taking an old machine of some sort and installing some flavor of lunix on it and turning it into a server at home to practice on. I want to use it as a file server, a print server, and maybe a sql server (I'm planning on learning SQL eventually as well as learning networking). Anyone have any suggestions of which lunix to use? Key thing is free for me. At the moment, I have 2 other machines that I'm using xp home on, and a third that's running vista home premium. Eventually, I will upgrade the xp machines to xp pro when I do have some money.