Talking about a small local ISP we helped put together, we used some relatively low-end pc's running OpenBSD+OpenBGPd on flashdist (openbsd read-only on DiskOnModule flash disk). This was about 3 years ago. The machine has a Celeron 2.4Ghz and 512Mb of RAM, and today, getting full routes from two upstreams, the machine is using 181Mb of RAM and itself routing about 300mbit/s of traffic ...
We run a similar network here. Only drawback at the moment is, that openbsd's openospf deamon does not yet support ospfv3, thus no ipv6...