After months of on-and-off fiddling about, I've finally got my home network accessing the 'net through Freesco on an old Compaq 486.
For those who don't know, Freesco is a floppy-based linux distro that can act as a router, ethernet bridge, firewall and limited http and ftp server.
Setting it up was quite a challenge for me, having very little linux experience and limited networking experience. Some of the documentation makes assumptions that you aren't flying blind like I was, though supposedly this is one of the easiest floppy distros to set up.
I managed to get it running behind my Linksys router, acting only as an ethernet router because I couldn't get PPPoE on Freesco to work, but had MTU problems and couldn't get any emails with more than a couple words of text in them.
Finally got it going without the Linksys at the "this either works or I quit the project forever" stage with the careful application of a home-made crossover cable and some tweaks to the dhcp ranges I'd set up for my two separate home networks. Very neat...I've got three cards in the box, one goes to the ADSL modem, the other two each go to a separate hub. It's dramatically cut down on the number of collisions from my former messy network (the Linksys's 4-port hub and two further hubs all stuck together).
For those interested in learning about this networking, setting this up is like taking a night-school course.
...and it makes good use of dumpstered computers.;)