From the numbers you have provided, you will play in league B of Moodle teams! I don't have first hand experience in installations of this size, so the following information is just speculation.
As Ken has pointed out, this is too much for a single server. The easiest way out is to use one machine each for the web server and the database server. If that is not enough, one can distribute the work amoung more machines, but it is more involved than the first step. See the ongoing discussion http://moodle.org/mod/forum/discuss.php?d=62896
and this http://moodle.org/mod/forum/discuss.php?d=60748
If you consider clustering this discussion has some starting points http://moodle.org/mod/forum/discuss.php?d=57202
One or two obvious facts:
- if you want to get the maximum from your hardware, forget Windows
- RAM is often the limiting factor. There is not much you can do to reduce the memory a Apache process/thread needs. There were suggestions to go for thinner web servers like lighttpd.
- a good PHP-caching system is a must
- after RAM, on the web server side CPU power is the next limiting factor, whereas the database server depends on fast disk system
There were couple of attempts to benchmark Moodle systems, to my knowlege there's not much to report.
Finally this thread has some pointers to old discussions http://moodle.org/mod/forum/discuss.php?d=50934