This link certainly saved me some time in evaluating the memory cost of modules and the core of Drupal 5 and 6. Although this is a specific case, I think the fact the core of D6 weights half the core of D5 is a considerable improvement and means that you can potentially serve a lot more users simultaneously on a high-load server:
http://2bits.com/articles/measuring-memory-consumption-by-drupal-bootstrap-and-modules.html
At the moment, I have 3 portals in mind that use more than 30MB of memory per execution and am figuring out how to improve this.