Did a little digging, based on the image above. Evidently it comes from the free utility Temperature Monitor, which, now downloaded, gives me the answer to my question: 65ºC.

I also see in my own case that there is a discrepancy between the numbers reported by iStat Pro and Temperature Monitor. E.g., on the dual 1.8 from which I am presently posting, iStat Pro reports CPUs A and B at 42º and 37º respectively, whereas Temperature Monitor simultaneously reports 38º and 37º. By mathematical necessity, both utilities cannot be right.

The 55ºC reported on the other G5 was in iStat Pro, so when back in the office, I will try Temperature Monitor instead.

But which to believe...?