In case anyone is following this thread, I thought I'd update y'all on my upgrade experience.
In short, it went amazingly smoothly, so much so I did it twice.
After doing a little more research and reading, I came across this article:
In it, the author offered the key to the puzzle, which was to boot from the Win8 disk itself, rather than running the upgrade from within the existing Windows install.
So my process was:
0) Purchase the Windows 8 Pro Upgrade disk from Newegg. They were running a promotion that made the physical product (which includes both the 32-bit and the 64-bit disks) only $13 more than the downloadable upgrade from Microsoft. Given that the only way I was going to be able to get a downloadable copy of the 64-bit version was to find someone with a 64-bit Windows machine who would let me commandeer it for a few hours, and the fact that 95% of my friends use Macs, I felt like the extra $13 was well worth it.
1) Physically install the new SSD into my desktop. This was by far the most time consuming part of the process for me, because I had to remove and re-attach my existing HDDs to make room for the new SSD and also to set them up so the power cables could reach all of the drives.
2) In the BIOS, I set the boot priority so that the DVD drive was first in line, followed by the new SSD.
3) I put the 64-bit disk into the DVD drive and booted up the machine.
From there, I was able to choose the new SSD as the destination for the new Windows 8 installation. I just let Windows 8 do its install thing, and everything was up and running in less than 30 minutes.
It was only after I got Windows 8 up and running that I realized I'd forgotten to switch the SATA mode to AHCI. I tried to fix it after the fact, but it quickly became apparent that it might be more trouble than it was worth. Instead, I decided just to flip the appropriate switch in the BIOS and do the install again (hence the "doing it twice"). Not a big deal -- the second time was even faster because I'd already set up a Windows account and I knew which options to select.
My primary concern after installing Windows 8 was whether the Windows 7 drivers for my aging M-Audio Delta-66 audio interface would work in Windows 8. I installed them and they work perfectly.
I've been using the machine for the past couple of days, and it's been completely stable (knock on wood).
All in all, I'm extremely impressed with how easy the install process was -- kudos to Microsoft for this, at least. As for Windows 8 itself, I can tell already it behaves better with my hardware in some ways than XP did -- for example, I could never get my machine to go to sleep properly before, but it works with Windows 8.
The UI, on the other hand, is kinda weird and unintuitive in some respects, and is going to take some getting used to. I'm going to try living with it for awhile before looking for fixes (such as bringing the Start menu back, etc.).