You're right, it caching won't help directly. But if every hit takes a tiny fraction of a second because it's (at least mostly) cached, it will take a lot more to overwhelm Apache. Caching Wordpress is not trivial, but it's not necessarily all that difficult -- it depends on the particular blog and how dynamic it has to be. I've had success setting up caching for my own stuff as well as a high-traffic online magazine.

Your larger point, BTW, that Apache can be overwhelmed, and that it makes a lot of sense to do some math and tune the configuration -- makes a lot of sense. I'd not really thought about that in depth before, but it seems obvious now. :-)