So I'm rocking the 15 inch Powerbook G4, early 2005 (1.67 GHz like yours, 1 GB of RAM, 80 GB HD). And I did some real world testing. Geekbench can only tell you so much.
These are fresh installs, no third party apps, OS fully patched and updated via Software Update
Boot up time with 10.4 - 2:31
Boot up time with 10.5 - 1:42
Start Safari with 10.4 - :10
Start Safari with 10.5 - :07
Copy 8 GB file to USB with 10.4 - 19 minutes
Copy 8 GB file to USB with 10.5 - 14 minutes
I think you see my point
Go with Leopard on that machine - It runs beautifully. You'll be kicking yourself if you don't.
These are fresh installs, no third party apps, OS fully patched and updated via Software Update
Boot up time with 10.4 - 2:31
Boot up time with 10.5 - 1:42
Start Safari with 10.4 - :10
Start Safari with 10.5 - :07
Copy 8 GB file to USB with 10.4 - 19 minutes
Copy 8 GB file to USB with 10.5 - 14 minutes
I think you see my point