Router with m0n0wall:
486 processor - Any 486 or higher processor is sufficient for m0n0wall. Exactly how much processor you will need for your particular implementation varies depending on your Internet connection bandwidth, number of simultaneous connections required, what features you will use, etc. For most deployments, a 486 or Pentium processor is sufficient.
router with IPCop
It will run on old, or "obsolete" hardware, such as a 386 processor, 32Mb of RAM, and 300Mb hard disk. But if you plan on using some of IPCop's features, such as the caching web proxy, or Intrusion Detection Logging, you are going to need more RAM, more disk space, and a faster processor