In principle you can use any computer based on an architecture supported by FreeBSD. For the examples in this wiki I'll assume you're going for i386 or amd64 since they're the most common. Although an old 486 class cpu would work perfectly for the basic router functionality, I'll give you some more reasonable recommendations here so you can make your router provide additional services.
- Pentium III 500 MHz
- 256MB RAM
- 20GB harddrive (adding an extra identical harddrive will allow mirroring which is highly recommended)
- 2 network adapters (Intel's are recommended)
- DVD-ROM drive for installation
- A standalone switch (don't use the built-in switch from your old router)
- You need to download the DVD image of FreeBSD 7.2 suitable for your architecture. The i386 version can be found here and the AMD64 version is available here. These images are gzipped so you need to unzip them before you burn them to DVD. In Windows you can use 7-zip for that.
- A free domain name from DynDNS or no-ip (DynDNS is recommended because they allow wildcards for free)
- Some basic knowledge of UNIX/Linux and console commands is preferred but not absolutely necessary
- Plenty of coffee or other non-alcoholic fluid to keep you awake...