My firewall is running mandrake 10.0. Its a Pentium 200Mhz with 64 megs of ram. Other than having to use text mode install (press F! when it boots from the cd) I dont remember having any particular problems with the install. its current uptime is 79 days and the last time it was rebooted was to upgrade the mandrake version its running.
The kernel image its running has the append statement:
append="noapic pci=noacpi devfs=mount acpi=off resume=/dev/hda1"
So you might need something like that given the age of the hardware.
If you install X on it dont expect much in the way of speed