G4 Required. The ZIF upgrade will get the system over the line but Apple has removed the kernel extensions for Gossamer and Heathrow so you will still only see a kernel panic.

You can pull the required kernel extensions from the Leopard WWDC previews or possibly from Tiger 10.4.11.

I've been receiving emails about the issue for the past 2 weeks, people wanting Leopard on beige G3's. Sadly it's quite an involved process.