Okay, I am still a "beginner" now approaching some kind of intermediate, but having these kinds of memory problems running PHP applications (notably several Drupal 6 sites through several Apache virtual servers all on the VPS 4 plan with 512MB memory) I finally came upon an answer: Run PHP on fcgi and then move to a threaded MPM. I am still getting it all working (a lot of little things change when moving from mod_php5 to the fcgi), but from what I am reading and now seeing, it is a way to keep Apache in a relatively low memory environment.

I will get around to writing up my experience and recipe in late November sometime -- I hope at nothinggoing.com

Hope that might help