An aside - are there any arm boards with dual Ethernet? Are there any with gigabit Ethernet? Could they theoretically even keep up with 2 gigabit ports?
If you are looking for something exclusively to do routing, you really should take a look at dedicated hardware from guys like Mikrotik http://routerboard.com/ or Ubiquiti http://dl.ubnt.com/Tolly212128UbiquitiEdgeRouterLitePricePer... - they're actually cheaper than the Sabre Lite and perform much, much better. They're also pretty open/hackable to boot.
You can install OpenWRT to compare although it looks like installation is a slight pain: http://wiki.openwrt.org/toh/mikrotik/rb450g - it's probably easier to install as a metarouter if you're just playing w/ it: http://wiki.mikrotik.com/wiki/Manual:Metarouter
its not arm its x86 I think, but you can put bsd and m0n0wall or pfSense on them. Also I believe these boards are designed for what you want to do and would do abetter job then a beaglebone.