As far as I am aware that's not possible on the G5 power macs?
That is my understanding as well. There's some resistor blocks that might help but the firmware or environmental control module detects when the CPU is running at a different speed than the motherboard is expecting and puts the whole thing into failsafe mode, fans blast at full speed and the CPU underclocks (say running a 1.8 GHz in a 1.6 GHz motherboard, underclocks to 1.3 GHz and blasts fans).
The only attempt I've seen is the one I just mentioned but it was using a stock 1.8 GHz G5 in a stock 1.6 GHz motherboard. No overclocking involved. The fans didn't freak out under LinuxPPC but the speed was only 1.6 GHz or 1.3 GHz regardless.
So if the bloke knows how to overclock a G5, I'm curious on a technical level. If it's just wishful assumptions, well, I feel ya, buddy.