Thanks all, especially for your encouragement about using the Terminal (I can remember circling around our family's first computer for about a year too).
I did do a disk repair from my original install disk with the Disk Utility, but unfortunately, my Tech Tool Pro 5 is nowhere to be found. Even trying to control the start up disk from the key board was courting disaster before I finally managed the first 10.6.8 update.
Alas, lightning struck quickly after trying the Combo Updater too. I had a crash with nothing but iTunes on deck, playing Foo Fighters while I occupied myself on the other side of the room. I returned to a flickering gray screen, which turned into a stalled, disintegrated screen saver when I moved my mouse. The music kept on playing though, till I moved the mouse again (although the cursor wasn't visible to me). That was that.
I shut down with the power button, then restarted and got a group of diagnostic messages including:
A Problem Advisory
A "Diagnostic and Usage Messages" Report generated for transmission to Apple
A Console Log
I'll post the Apple report in a separate comment, assuming it's the most relevant. Should I post the others the others too, for your (kind and patient!) inspection? I've got a couple photos of what Apple's Cosmos screen saver ended up looking like, but I'm not sure whether I should expect the actual images to show up in the Preview window, rather than just the URL with image tags, so I decided not to muck things up unnecessarily.
Here are a few of my particulars.
Model Name: Mac Pro
Model Identifier: MacPro3,1
Processor Name: Quad-Core Intel Xeon
Processor Speed: 3.2 GHz
Number Of Processors: 2
Total Number Of Cores: 8
L2 Cache (per processor): 12 MB
Memory: 32 GB
Bus Speed: 1.6 GHz
Boot ROM Version: MP31.006C.B05
SMC Version (system): 1.25f4
I have a nagging feeling that my Wacom Tablet, which refused to sleep in under Tiger & Leopard may be exacerbating, if not creating, the problems. My driver is up to date, though. Could the fact that I'm using a 32 bit display be a factor?
NVIDIA GeForce 8800 GT:
Chipset Model: NVIDIA GeForce 8800 GT
Type: GPU
Bus: PCIe
Slot: Slot-1
PCIe Lane Width: x16
VRAM (Total): 512 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x0602
Revision ID: 0x00a2
ROM Revision: 3233
Displays:
Display Connector:
Status: No Display Connected
Cintiq21UX:
Resolution: 1600 x 1200 @ 60 Hz
Pixel Depth: 32-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
