Can't find a schematic for the powermac G5 motherboard, so I got one for a similar G5 based computer (iMac G5?). Page 30 is the CPU Straps Page, it lists pll & multiplier settings. The CPU is codenamed "Neo" which is the same processor as in the Powermac G5.
There's another chart which lists multiplier settings, PLL settings, and the resulting speed.
I don't have a Powermac G5 to experiment with, unfortunately.
Thanks for all I'll try to look into this, sounds like a fun way to spend the day off.LOL