DD-WRT is an extremely popular firmware upgrade for wireless routers, and for good reason. It’s extremely powerful, and allows you to use a cheap wireless router to perform the function of costlier hardware.
A commercial wireless router takes up a lot less space and consumes a lot less power than a PC, but sometimes you might find yourself needing a router for a short period of time. You could go spend $50 on a router, but if you have an obsolete PC and a pile of NICs laying around, why not just press that pile of junk into duty to get the job done and save 50 bucks?
That’s what DD-WRT x86 lets you do.
Even a Pentium II with enough PCI NICs to give you the network ports you need would make a capable router. Routing isn’t terribly complicated work, and routers tend to have pretty modest specs–200-300 MHz CPUs and 32-64 MB of RAM. They tend to have very modest storage too. A 40 GB hard drive is pretty cramped for general purpose computing–the equivalent of a cramped room at the HoJo Motor Lodge–but for a router, it’s mansion-like.
DD-WRT’s site has installation details, but basically you install a second drive and some extra NICs, boot whatever operating system the computer already has and use it to write the image to the second drive, then boot off the second drive. Plug the first NIC into your network, plug at least one PC into one of the secondary NICs, point a web browser at http://192.168.1.1, and configure the router.
It’s not quite as easy as taking a router out of the box and plugging it in, but I remember spending entire weekends turning 486s into routers in the late 1990s. This is more like a 15-minute job, which is about how long it would take, best-case, to hop in my car, drive to the Office Max store a couple of miles from me, buy a router, and come home.