You say it is a 400MHz machine and your signature says 440MHz....
Go to apple.com and to the support page. Find something to enter your serial # in. The serial number is found on the back of your machine or in about your mac from the apple logo in the menu bar.
do this so we can tell you which model you own. I assume it is the gigabit Ethernet model.
Go to macsales.com and find a processor for your model.
We are not at your disposal 24/7. Give us more than 3 hrs to help you.
Your machine may be upgradeable with a CPU upgrade from Sonnet, OWC, Powerlogix and other upgrade manufacturers, and you may be able to install Leopard.
Go to OWC and see their list of upgrades, but they ain't cheap..........................
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.