A small follow up. The second processor in this Power Mac failed last night, and since the Early 2005 systems require both processors to be present for the machine to power up, it's currently out of commission. I'm not sure whether it will be repaired, depends how difficult it is to find another Early 2005 2.0GHz processor (and yes, it has to be from the same year otherwise the machine won't POST).
I have the malfunctioning CPU out, and it appears to be a bad solder joint on the card somewhere toward the connector end, so I'll investigate repairing the existing processor card. Some other options include attempting to repair the U3 on the stock June 2004 Logic Board so I can slot in the factory 1.8GHz Dual CPUs, or installing a spare Single 1.8GHz logic board and processor I have sitting around.
Rather disappointing since it looked like it was going well for a while there.
That is really unfortunate... I was going to strip my G5 down and do all new paste, remove the PSU cover and blast the dust out and all that, but it is my daily work station and I don't really want to risk it too much.