The faster models come with a 600 watt power supply. Thats almost twice the output of a G4 power supply.
I'm not sure about PA Semi's product, but I'm fairly sure it uses less power.
Some PowerMac G4 are really sucking a lot of juice. But don't just trust the PSU ratings - do actual measurements. The 2x2GHz G5 Powermac drains about 150 on average use. That's quite a lot. But I was speaking about the iMac G5 - and these don't take that much enrgy up. Just read my numbers from above.
So it is true, there are PowerMac G5s that peak at max load with really high values (in the several hunderet watts range), but not *all* G5s. Select your G5 with care and then these issues become void.
I use 100% renewable energy, a kW costs me about 0.215 EUR and I am quite employed on energy efficience and stuff like that. I am pretty much aware of what I am talking about.
Have to check the numbers of the iMac 20" myself though (don't have one yet). Same is true for the X1000. IIRC in some posing it was said it sucks 25W - sounds plausible to me.
Given the numbers I gave above are right, an iMac G5 2 GHz consumes energy for 10EUR more on a year than an X1000... Speedwise these devices should be pretty on par.