OK, so I finally got a side box up and running. I had a SuSE distro lying around that I got way back when (planning, at the time, to put it on the primary box), so I installed it. Everything's fine but I'm not getting the network up and running.
Card = 3COM 3c905b-TX
LinkSys Cable/DSL router (BEFSR41) serving as a DHCP server
In network setup, I chose 3COM 3c90x card, and set to use DHCP client to get IP address. There's no apparent errors in the kernel startup on subsequent boots, but I can't ping the router.
I'm certain that the hardware is all working correctly, as I've been able to boot Win98 on that machine (and access the net through the router) with no issues.