Almost forgot to report back. APC  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 , dated 2008, reported an improvement by a facter of 3 for Drupal.
Hardware and performance
Update on PHP-accelerators
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