The new motherboard, CPU, and memory arrived yesterday, and snapping them all together was surprisingly simple. The new motherboard is from the same manufacturer as the old one, so it was also a simple matter to note where everything was plugged in on the old board, then plug it all back in on the new one. Also, the case has a slide-out tray for the motherboard, making it pretty simple to switch them out.
Of course, once I got everything set up and turned it on... it showed the exact same problem as before: everything powered up, but nothing actually happened. At this point, I tried buying a new power supply and trading that in -- still nothing. I disconnected all the devices from the motherboard -- still nothing.
And then, finally-finally, I removed all the case connections from the motherboard except for the power switch. THEN IT WORKED. From there, it was a process of elimination; I finally worked out that the case's reset switch was somehow busted. If I left it disconnected, everything worked fine.
Just for grins, I tried hooking up the *old* motherboard/CPU/memory combo with the reset switch unconnected... and that *still* didn't work, meaning that yes, I had a busted motherboard, and I wasn't just wasting $200 on replacement equipment. I suppose the motherboard shorting out and the reset switch becoming problematic must have been related.
I tried leaving the existing hard drive in the machine; to my surprise, XP started up pretty normally. I just had to install the drivers for the new motherboard (which was a bit of a challenge, since the OS didn't yet recognize the USB mouse). I'm holding off on spending more money for the moment, but eventually I'll upgrade it to Windows 7 so that it'll use all its cores and its memory.
Then there was a bit of a scare when I tried using the new BIOS's "core unlocker". The triple-core CPU I bought actually has a fourth, disabled core built in, and you can try activating that. Sometimes the fourth core works just fine. Other times, as in my case, the fourth core is wonky, and it makes XP un-startable. I reverted the motherboard to factory settings, and now everything seems happy (fingers crossed).
Meanwhile, I've added an XP install to the iMac, using VirtualBox, which solves the problem of "how can I find good replacements for Notepad++ and AutoHotkey on this machine?" So far this looks like an ideal solution: I use the virtual XP system for all my day-to-day text editing and web stuff, and then I can switch out to Lion for the mac's far-superior multimedia applications. No dual-booting required.
I'm growing to hate the Magic Mouse, though I love the *concept* of a mouse whose surface is a multitouch trackpad. MagicPrefs makes it tolerable. I finally managed to shut off horizontal scrolling, which is just terrible, especially since the default *behavior* for horizontal scrolling seems to be "scroll vertically really, really fast". And I managed to get back the "middle button does a double-click" behavior that I've had Autohotkeyed on Windows since forever. But even then, the device feels wonky and imprecise.
I may go ahead and replace that mouse. I've already replaced the keyboard (which I couldn't stand, although it was very pretty) with a proper ergonomic one.
Anyway, hopefully that's it for fussing-with-computers for the time being.
 I would call this "the best of both worlds", but "Ship in a Bottle" feels like the far better ST:TNG metaphor. #computerendprogram
 "Yes, I'd like it so that, whenever my hand brushes against the mouse, you scroll to some random part of the page. Can you do that for me?"