A lot has happened since I last posted. I purchased that "SuperGrace" YMF724F-V sound card with the 3.3V notch, and it didn't work, most likely because the YMF724 requires a digital 3.3V supply but 5V for I/O.
I looked at the datasheet for the YMF744, and lo and behold, it supports 3.3V for both supply and I/O. I kept looking for boards with the 3.3V notch, but didn't find any. Then I read about this daring person who cut the 3.3V notch into a different PCI card to put it in a 3.3V-only PCI slot, and it worked there.
So I figured, it's worth a shot. I bought an AOpen AW744L-II new-old stock off eBay and used my dremel to cut out the notch. And guess what?
Yeah, it still doesn't work. Unlike the SuperGrace card, which just wouldn't initialize properly, putting this notched card into my board bizarrely breaks the Primary master IDE device (not the slave) and the PS/2 mouse for some reason, in addition to freezing Windows 98 when it tries to install the device. I'm really confused as to how and why that happens.
Looking at the board, it has a Sigmatel STAC9708T codec, which naturally requires a 5V analog supply. Didn't notice that before. The otherwise identical Sigmatel STAC9711 codec is the one which takes a 3.3V analog supply. It's not clear to me how that would cause all of the weirdness on the PCI bus that's causing my computer to freak out.
So close, yet so far. The codec is a SMT device, so it's theoretically possible to swap it out for its 3.3V cousin, but the pins are very fine pitch and I would have to get a soldering expert's help with that, assuming I can even source the IC.
All that said, does anyone else have any other ideas why this AOpen board is causing my system to misbehave?