thanks unc, I'll check it out.

I have the Service Source manual for the June 2004 G5 which was very useful.

The CPU's are secured by those 2.5mm hex screws; I got a T-handle wrench with a 200mm blade, and that's the right tool for the job. The screws are tapered just under the head and spread what is effectively a collet with a knurled surface which grips the 1/4" aluminium base plate of the whole CPU/Heatsink assembly. Loosening the screws simply relaxes the collets and the CPUs can be lifted off the motherboard. Those standoffs with the collets machined on one end are screwed into lugs on the side of the G5 case, and are hex-barreled at that end so that one can unscrew them with a wrench. The lower pair of standoffs have to be removed from the case so that the PSU can be tilted out of the case.

The Service Manual take-apart assumes that one has already removed:

Front inlet fan assembly
Heatsink cover(s) for Power Mac G5 or
Heatsink cover(s) for Power Mac G5 (June 2004/Early 2005)
All AGP and PCI cards
Rear exhaust fan assembly
Front panel board cable
Memory DIMM if it is in slot next to the power supply cable

With the AGP/PCI cards removed, the power cable for the rear exhaust fans is easily removed and reinstalled.

The manual's processor removal procedure includes removing the standoffs, but only the bottom two need to come out.

The cover plate screwed onto the PSU needs to come out.

There is very little clearance available to tilt the PSU and lift it out of the case, but it will come. If the harness to the PSU's P3 connector (the small one at the front of the G5 case) lies between the PSU and the side of the case, the PSU won't come out! I had to push it clear with a screwdriver.