Currently, the NS2 Server software is not fully optimized to make use of multiple cores on your system. At best, it uses 2 threads, of which 1 will handle the main processing.
As such, NS2 Server providers commonly run their servers on overclocked consumer-grade systems as opposed to multi-core server-grade systems. e.g. NS2 Servers which handle 24 players are typically clocked at 4.4GHz on an Intel i7 core.
Vanilla NS2 servers take between 500-700MB of RAM upon starting the Server, as it caches the animations and models. 2GB of RAM per server instance is a good amount to have.
Ouch, might be a bit too demanding, although I am unsure of our setup.
http://www.unknownworlds.com/ns2/wiki/index.php/Dedicated_Server