Thanks for these replies. The chips are identical, and therefore I assume that the VRMs are, too.
The support info on HAL was something I knew nothing about, and frankly, at first glance it's way above my head. On second glance, however, it says,
"On Windows XP and later versions, the ACPI Uniprocessor HAL and the MPS Uniprocessor HAL recognize the existence of more than one processor and report the MP ID. Plug and Play detects that the computer devnode's hardware ID list has changed and moves the devnode back through the "found new hardware" detection process. Therefore, when you add a second processor, the MP files (HAL and kernels) are automatically installed, and you do not have to manually update the driver in Device Manager."
Sysinfo32, under hardware resources, IRQ3 device is listed as "Microsoft ACPI-Compliant System." Is there another way to verify it?
Are you sure I'd have to reinstall XP? Am I missing something? It looks as if I choose the last listed HAL type. I don't understand the difference between UpdatUPHAL and UpdateHAL, but if the machine is ACPI going from uni to multi-processors, it looks as if the change is automatic.
As I said, I don't know how my applications would use the two processors. The most processor intensive stuff would probably be the Silverfast scanning software, plus whatever image editor editor I'm using on fairly large TIFF files. Other than that, I don't use a lot of power: some audio editing uses large files, perhaps, but it's because of the photo work that I'm thinking of adding the other Xeon, which is indeed collecting dust.
I've been using XP for a while on the 235, so I have a lot of programs installed, and it would be a bit of a bother to reinstall them (though the system probably would benefit from the maintenance). The 235 server has four smallish ultra-SCSI drives, and I can fairly easily have one remain for booting the current configuration if I have to go back. Or am I risking a total mess from which I won't be able to recover.