In general for small hardware I go with two options. Qualcomm-Atheros AR9331 based systems, with 64MB ram and 16MB flash (kind of hard to get in that config cheaply). You can get decent performance, potentially hang a 5.8/2.4ghz radio on it too, and power it off a single laptop USB port (200mA normal power use, and with stuff, still less than 500mA.) Can put second ethernet, wifi, 3G, etc. there. There's no current c…
It would be a bit higher cost, but could mean you could make a router like the EdgeRouter but with good performance without the proprietary accelerator (I believe that Intel's drivers are open source).
1. Data Plane Development Kit, which lets you skip the kernel IP stack (which takes thousands of CPU cycles to process) and do packet processing in userland taking just tens to hundreds of cycles per packet. http://dpdk.org/