Forget the number of servers, the "firewalled" bandwidth is an issue here.
If you like Cisco, you can go ahead with their Cisco PIX firewall (be awared, you can make your own Cisco PIX firewall from an old Intel BX440 motherboard + flash memory, but this is another story)...
A lot of vendors offer firewalls, based on a simple PC with Linux / FreeBSD, that is a fact.
You will need a real hardware / embedded solution for high traffic - above 70Mbps.
If you would like to plan for growth/scalability and have flexibilty as well you might want to consider the Checkpoint product line. This firewall can protect you all the way up to the application layer and is way more sophisticated than a port based firewall like Cisco PIX. The only firewall in a comparable class is Netscreen.