i don't know about firewall, since a good recent modem will have a built in one.
i set up my P3 as a family computer, for my nephew and niece to play on, and leave my c2d alone.
i had a P1 computer that ran a live cd arcade emulator on it. it's fun for a while, but there aren't any other good uses for it. it was a 266mhz, 128mb ram and had a 16mb video card. ok for playing frogger, pacman and other classics.
you could run linux, but even the lighter ones like dsl, puppy, etc run a little slow, for me at least. depends how much you want to get into it. for linux i find it better to run one as virtual machine on my current setup. it's faster and you don't need extra monitors, keyboards, mice, etc.
since you already have plenty of good computers, i would suggest getting rid of the old inventory so you have more space when you do upgrades on your current computers.
i god rid of three old hp pavilions that i had lying around. got $40 for them but now my closet looks nicer