IPCop has very modest minimum hardware requirements and is designed to be used on common PC hardware. Our main issue generally isn't "is this machine powerful enough for IPCop" it is more a case of "is this machine powerful enough to handle the bandwidth going through it".
The minimum system requirements for IPCop are:
A 386-based PC or better (which means you can use a very old PC as your firewall)
32MB of RAM
200MB hard drive (or compact flash drive)
One Network Interface Card (NIC) for each interface (four maximum). Each card has a unique hardware or MAC address, it's worth noting these down at this point for use in identifying the cards later.
Connection devices for the Red interface if it's not Ethernet