www.linuxiso.org is a great place to download Linux distrobutions...
there are several ways to network computers (in order to share the net connection), you're going to need a hub or switch if you have more than one computer to share with.

I'm building an old 486 (66mhz) with two NIC's (old ne2000's) and a 100mb HDD (i'm using FREESCO) this is what i'm doing to share my i-net connection.

As far as what version of linux (don't forget BSD ) it all depends on what you want to do with it... considering the speed of the machine, i would consider running a minimal Linux setup...

-take it easy...
-tampa