yeah, about a jillion. seriously, if you have enough users that the internet link is getting bogged down (12 per cable line, prolly) then everyone will complain. unless you have a 10mbit line, and 1000 users, it's probably not gonna be a problem.
I run FreeBSD 4.10+IPFW at work between a web site that routinely pushes out 20-25 megabits/sec of traffic and the rest of the internet with no problems. The system is only a PIII-750 with 256 mb of RAM, which is way overkill for the stateless packet filtering its doing.