Today, I notice that my last reply never reached the board!
EDIT: entering this new post, I notice that my last reply was posted! Oh well, no harm done.
Let's summarize:
1. after a complete reset as fryke described above, the Airport Utility on my Intel iMac recognized the Airport Express and I could start configuring. First, I did end up with two different networks, but I got them joined to one and music streaming again from the Intel iMac to my stereo. Success & thank you, fryke!
2. after a couple of failures with my printer, I gave up and decided to keep it connected to the Intel iMac, at least for the time being. Work done on the G4s will be transferred wirelessly or through a USB stick to the Intel iMac and printed from there. Not exactly what I had planned upfront, but for the time being the best I can do.
3. my two G4's (iMac and Powerbook), both running 10.3.9, can join the wireless network, but I have to connect manually, although as far as I can see, they have been set to connect automatically. Not a big problem, just a bit of a nuisance. Can this be fixed?
4. my iMac G3, running OS 9.2.2 can no longer join the wireless network, because I switched from WEP to WPA. Don't even know if one still has the WEP choice under Leopard? Not a big problem, since this Mac is in fact only used for LP-to-CD conversions, and I can always transfer through a USB stick.
5. music streaming from my Panther Macs is interrupted, something I vaguely remember from the past (it was related to that pesky iTunesHelper, but I haven't seen or experienced that for ages). I will update to the latest iTunes and Quicktime and try again. Again, it's not a big problem, because I can stream flawlessly (be it only from the Intel).
Finally, a question: does anyone have a suggestion what to do with the Airport Extreme Basestation (the snowy white flying saucer) which is now sitting unemployed in my cupboard? Is there any way I could integrate it into my new wireless network, or should I just pass it on to my son?