Well although you can use arch to be a router, its probably not the best route.
For a router you usually want many Ethernet ports. These are usually hardware designed specifically as routers. Most ARM SBC only have a single ethernet port. Some two ports, and a very few 4 ports.
Routers, even cheap ones tend to have 4+ ports, are very cheap, but limited resources, both memory and flash disk. This said though most run linux. If you want to get access top the linux on the device then
https://openwrt.org/ is the direct to look. This is designed specifically for very limited resource routers, that are primarily routers. As its design for this, it sets up routing very easilly; and well. So its a good direction to look.
Its not to say that you can't do routing using Arch. My NAS backup has arch on it. Connected to it via its usb ports are a small farm of beagle devices (well two actually); the beagles bring up an ethernet port on the usb. The arch on the NAS then routes for them, as its directly connected to my ADSL router than connects to WAN.
Now I've spent far more time setting up routing on the NAS than the router (Router has openwrt installed), and the routing is less capable (havn't yet got IPv6 to work); but was an interesting learning experience.
So anyway, my advice if after a router, is first take a look at openwrt forums. See if that offers what is needed.