It's not a recommendation, its a requirement:I have a Power Mac G4 and would like to upgrade to the latest OSX. Apple recommends 867 megahertz processor.
Apple said:Mac OS X Leopard General requirements
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
Upgrading the CPU is a whole different ball game - not nearly as easy as upgrading RAM. My advice would be only upgrade the CPU if you absolutely know what you're doing. You could pay someone else to do it but then is it worth it?