First, a note of explanation. My current setup involves a cable modem plugged into a linux box that acts as a firewall and router, which connects to the rest of the home LAN (which varies between half a dozen and a dozen machines).
Now, two of the machines well, both when accessing the Internet, and as I later discovered, when moving files between machines on the LAN. I was also rather worried when I got 25% packet loss between two machines sitting next to each other on the same LAN. This started to give me the impression of either a bad network card on some machines (less likely) or some other network connection issue. So, for kicks, I pulled out an old switch that I had laying around (I'd actually been meaning to replace the hub with it for a while), and swapped the hub for it.
Seems to have done the trick. ;-)