Synchronize drupal database with a traditional database
Easy answer: use the data module for drupal 6.x

http://drupal.org/project/data

"Data module helps you model, manage and query related sets of tables. It offers an administration interface and a low level API for manipulating tables and accessing their contents. Data module provides Views integration for displaying table data and Drupal search integration for searching table content."

The included data_ui module will let you setup the connections to your external database and map the fields.