Unfortuantely, even the fanciest boxes running BSD can't complete on a pure throughput basis with good Cisco routers. An twenty-four port gigabit Cisco router has a 48 Gbps backplane, but a PC running BSD will be limited by its bus--the fastest servers have a 64 bit 133 MHz bus with PCI-X. That's 8 Gbps. And you can't put more than a handful of network cards in even the largest BSD-capable server--there simply aren't the expansion slots. So this really couldn't be used for core Internet routers.
I think you may be confusing switches with routers. Cisco has some nice switches like the 3550-48. These switches contain basic routing capabilities. The Cisco switches work well with BSD routers, and OpenBGPD fits in here.
If you are talking about Cisco 10000, and 12000 models, then it's a totally different ballgame. These things when fully loaded cost more than most houses. They're generally limited to full-on service providers, not medium sized businesses with 500 employees.
