As dtravis7 suggest first make sure firmware is up to date, probably version 4.2.8f1 for your machine. If you do not have the update already installed for running OS X, it needs to be downloaded and installed whilst still in OS 9.

On the Sonnet site there may be a downloadable video on the install of the upgrade card. Just a matter of removing the heat sink clips, sink and undoing three screws on your current processor, and replacing with the card and replacing screws and heat sink and power lead/s, if any.