Whew! By dint of perseverance and lots of plugging and unplugging, I've got the machine virtually back as it was on Wednesday before I started. The BIOS for PCI3 is "locked" to IRQ5, the sound card is in PCI3, and CP says the sound card is on IRQ9 like everything else.
I've only got one SCSI card in there now, so I don't know how CD writing will be - it used to object to having a scanner on the same bus. If it still does, I'll put back the other SCSI card for the scanner.
Skybound still doesn't appear anywhere when I plug it in. Might be faulty!
Still not got the "upgrade" of XP - the Post Office is on strike