I didn't know that IPCOP could run on one that old. I have one like that up in the attic, time to bring it back down. Before I upgraded to 5.3, I was running 4.7 with FireStarter and did not have any troubles. As soon as I get some sleep I will be looking in to setting it up.
If it is a pure firewall/nat box then you may want to give OpenBSD a try. Expand your horizons a bit. I ran OpenBSD headless on a Pentium too but with a bit more RAM and diskless too.