then I'd highly recommend a unix or linux box as your user load startst to scale. If you want support, we've had pretty good luck with Red Hat Enterprise (better than M$ or Apple, certainly.
MacOX is a great OS, but it falls apart at high user loads, esp. when running a database.
Rather than an OXServe, get a dual xeon or opteron (the real geeks here say opteron, get 4GB RAM, RAID if you can afford it. You should be fine up to ~10k users on that, though it is always a good idea if you can afford it to run the database on a separate machine from the Apache/PHP server, or start moving toward a real cluster if (multiple Apache/PHP servers) if you are going to have high concurrent access. For a 'normal' LMS load (~10% concurrency), a dual xeon should be fine up to ~10k users.
It is important to know the network connectivity, as that can cause a bottleneck if you max out your network, that has nothing to do with the server hardware.
If you are going to do alot of video or audio, keep the Mac server as a video/audio serving machine, that is what we did with our old Xserv that we used to run Moodle on), it serves fine for that purpose and helps take the load off the main Moodle server.