D6 Is it possible to set node reference field and use this value in another field on the same node/add form?
These two modules are intended for that purpose and have D6 versions.  I think Dependent Fields sounds closer to what you want, but Hierarchical Select Node Reference may work almost as well.

Dependent Fields


  The Dependent Fields module (formerly AHAH Dependent Fields) allows cck based fields to specify another field as a controller field, and react to its changes on the form using AHAH/AJAX without the form re-loading.


Hierarchical Select Node Reference


  This module allows the use of Hierarchical Select form element as a widget for CCK node reference field, both in node add/edit form and in views filter.