To downgrade to Jaguar, you would have to do a clean install. But you will not have the Jaguar discs (as your computer comes with Panther) so you will either have to purchase Jaguar (stupid idea, and that's if you find it, although Amazon.co.uk is still offering Jaguar for the same price as Panther!!!) or get it from someone you know. That would be a grey area legally but since you own the licesnse for Panther, Apple will probably not care.
Do you really want to downgrade? I've heard of some problems with 10.3 but I'm still waiting for the upgrade! I'm sure 3.2 or 3.3 will provide most of the fixes you need.