To get a more acurate measure of real Moodle/Apache thoughput we have used the WebLoad tools. (see http://www.webload.org/) The tools tell us exactly how man concurrent session are being run. We can then throtle more concurent sessions until we make the server go belly up. A lot of thought must go into the session scripts to make the data valid and a reflection of real world use. There are also some Moodle settings that must be re-configured. To be able to test down to the quiz level, automatic randomizations of questions and quizes must be turned off. Each session script has to be added to provide a unique logon id, and all the logon ids must be pre-enrolled in the course under test. The course may need to be reset between each load test. The session scripts are simply a keystroke record of a session where you simulate the use you are trying to test. The session sciprts then can be edited for a large number of users and various delays can be inserted or removed to better simulate the mix of users. The start up and loading of the sessions can be timed or programmed to simulate everybody jumping on the machine at once or for a closer to normal system usage.
Thanks to the data from our testing we were able to tweek the OS and Apache configs to host about 1900 sessions before we brought our server to it's knees, so to speak. [server info: Dell PowerEdge 2950 32GB memory, 4 64bit cpus, direct attached scsi disk 400GB separate LDAP server, separate mysql server] We did some runs only for log-in testing strictly to test the authentication server.
Which ever way you choose to do load testing be prepared for some long hours.