Sigh. I'm reminded of a friend of mine who told me that "things always takes longer"...
My wireless installation is currently only a 33% success - at least the AP is up and running. But neither of the two PCs will boot with the PCI-cards mounted. One box won't even give me the BIOS start-up screen, the other gets as far as the "DMI pool"-bit, where it hangs.
Rummaging around on D-Link's support site, I found a place where it says that the card "requires a PCI 2.2 compliant motherboard (3.3v) in order to function". Both my motherboards are old surplus Pentium 100/133, so I bet that's my problem. I've got another, newer PC, where the cards works flawlessly. But that PC is sitting right next to my ADSL router, so doesn't need WiFi.
I'm not sure which way to go now - probably go out and get at least one new motherboard, upgrading one of the PCs to a status where it can use the PCI-card. Then maybe I can swap the other card for another AP, and use that in 'Wireless Client'-mode with the other PC.
Talk about throwing good money after bad...
Update: This ended up being what I did, and it works fine. If you're interested, mail me, and I'll describe the final setup.