Drupal 6 as a call logging system - question about node performance & scalability
This is quite a subjective question. Your best bet would be to ask on some of the Drupal groups that handle this stuff like Drupal High Performance.

There are so many factors in making a drupal site scale it's hard to list them all.

Pressflow is a drupal distro that is built for scaling, they also have some great blog articles on scaling.