Well I had no idea that the work a node does is so intensive, I always thought the miners are doing the heavy work and the nodes are just picking up those few kbytes of traffic and adding it to the chain...
Acting as a relay node is only hard work for something with such low computing resources as a typical router. Bitcoin verifies the signatures of transactions it receives from other peers, this is hard on a processor that is chosen for use in a modern router. It's possible that a more specialised router might be available today that could handle it. I think something with a 1 GHz processor and 1GB RAM would be comfortable. Maybe there's something out there that can make use of that specification.
A Raspberry Pi or a Beaglebone class of device can just about do the job, as they have versions with 750-1000 MHz processors and 512MB RAM. From my experience with the Pi, it's still a bit stilted with only one processing core, you can't really do anything but the most undemanding tasks while the bitcoin client is running (without heavily impacting the responsiveness of the machine).