"Hey Dave!! I've checked out that you own a cpu upgrade as I do, and you are using mac os x 10.5.6. Did you have any problem using your upgraded g4 with it?"
No problems.
"I was wondering if there is something I must do before installing the new os, so that the cpu is recognized."
Depending on your processor speed, you might need to load Gigameter from your hard drive when installing Leopard. I didn't need to because my upgrade runs at a 10x multiplier.
This chart shows which speed settings are recognized correctly by the system, and which ones will require Gigameter.
http://eshop.macsales.com/tech_center/index.cfm?page=extremesettings.html
There are instructions for loading Gigameter from your hard drive here.
http://www.brad-x.com/2007/10/27/howto-leopard-install-with-giga-designs-g4-upgr ade
"I'm assuming that the kext extension installed by Gigameter should be reinstalled after the OS upgrade"
Yes, if needed.
"and that the firmware won't be touched by Leo upgrade"
I haven't noticed any changes, and I can still boot from OS 9.