I am looking for some wisdom on configuring APC on a LAMP system.
Links to good reference material would be helpful as well.
Probably just about all of them.
It's impossible to give a more precise answer without a detailled understanding of the workload and the structure of the sites.
But even then tuning is as much an empirical art as a science - you need to have good systems in place for gather and analysing metrics then comparing models.
I don't think that there are many hard and fast rules, will depend case by case. To start off, I would enable APC with the default settings, and see if there is enough shared memory allocated to store your compiled pages, and any other values you want to store. If you start running out, tweak and repeat.