Hardware and performance

Almost forgot to report back. APC [1] is simple to install under Debian "Squeeze", easy to configure.

The installation could be as simple as
# apt-get install php-apc
# zcat //usr/share/doc/php-apc/apc.php.gz /var/www/moodle/admin/
(The webpage http://your.site/moodle/admin/apc.php will display the APC INFO.)
# /etc/init.d/apache2 restart

The configuration is through /etc/php5/conf.d/apc.ini (don't forget to restart apache).

There's page of its own for APC and Windows: http://docs.moodle.org/en/Installing_APC_in_Windows

I have not done any direct comparison, with and without APC. This benchmarking [2], dated 2008, reported an improvement by a facter of 3 for Drupal.

[1] http://www.php.net/manual/en/book.apc.php
[2] http://2bits.com/articles/benchmarking-drupal-with-php-op-code-caches-apc-eaccelerator-and-xcache-compared.html