I'm looking for a set of servers for performance testing a network, and secondarily testing some applications on the servers. Their most important task is simply to pump out data: from an application like memcached or just dumped from a large file in memory into a TCP flow (i.e., disk performance doesn't matter). This should happen over one or more 1 gigabit Ethernet ports, and the machines should run Linux (ideally), or perhaps Mac OS X or some other *nix. Other than that, there are few constraints (e.g., even something ARM-based could be fine).

So here's the question: What's the cheapest server per gigabit? Price and power are both considerations.