First, there is a little more than just putting Linux on a Pentium box to make a firewall. You will also need some firewall software to do either proxies, port-filtering or both.
As for the load, a lot depends on the bandwidth of the data. Are you running these six servers on a 10MB Ethernet? 100MB? Connected by fiber? The more bandwidth, the more data that has to be processed by the firewall. The firewalls have a lot less processing to do than the servers, so generally a 266Mhz Pentium could serve more than one server. However, you'll have to experiment to determine exactly how many. Or, you can hire a network engineer that can do the calculations to figure it out in advance.
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This was first published in February 2002