There is now download available to take you from 10.3.x to 10.4.x

The move to 10.4 is a paid upgrade which you may find at the Apple store or at Amazon.

Once you have 10.4 installed, you will find that Software Update will download the necessary software to take you to 10.4.11, the highest 10.4 went to.

Usual price of 10.4, at Amazon is around $150. The higher price is because it is no longer in production.