http://www.google.com/url?sa=t&source=web&ct=res&cd=17&url=http%3A%2F%2Ffreedomhec.pbwiki.com%2Ff%2Flinux_ip_routers.pdf&ei=yXDzSdjSD4yYtAPG_cXPCg&rct=j&q=linux+routing+performance&usg=AFQjCNGNHX6jWJ7Ybrg8Jod24MMenJYt0g

Paper from vayatta with some benchmarks that shows raw ip forwarding performance with various packet sizes and % of packet losses. Basically 2.8 Intel celeron + 1G broadcom (tg3) cards can deliver ~150Mbps on 64b packets limited by CPU