> wasn't it that only ONE ever existed, in the hands of Gunne Steen, of GGS Data?

The only one ever existing "in the wild", yes, I think so. He even ran the web server for his shop (and pegasos.org?) on that machine.

> we'd better forget about adding support for dead old AGP cards. By the time you
> reach it, the cards will be impossible to obtain. Let's support current PCI-e cards.
> Here's one. What? Where'e the thing this plugs into? Solution?

Radeon gfx cards vendors usually have been providing some of their PCIe based Radeon GPU based cards also with AGP and even PCI connectors (via on-card PCIe-to-AGP/PCI bridge obviously). For instance these:

http://www.hisdigital.com/un/product1-47.shtml (AGP up to HD4670 (RV730))
http://www.hisdigital.com/un/product1-48.shtml (PCI up to HD4350 (RV710))
http://www.diamondmm.com/AGP-Video-Cards.php (AGP up to HD4650 (RV730))
http://www.club3d.nl/products/product_graphics_1.cfm (AGP up to HD4670 (RV730), PCI up to HD4350 (RV710))
http://graphics.visiontek.com/video/3000/3000.html (AGP up to HD3850 (RV670))
http://graphics.visiontek.com/video/2000/2000.html (PCI up to HD2400 (RV610))
http://www.sapphiretech.com/presentation/product/?cid=1&psn=000101&gid=4 (AGP up to HD4650 (RV730))
http://www.powercolor.com/global/products_search_VGA.asp?Bus=AGP (AGP up to HD4670 (RV730))
http://www.powercolor.com/global/products_search_VGA.asp?Bus=PCI (PCI up to HD4350 (RV710))

> Choose computers with built-in graphics functions, like the Mac Mini. What a
> pleasant coincidence, it even has a familiar Radeon chip...

I fail to see the advantage regarding GPU support there. Any computer without built-in graphics functions but with AGP or PCI slots could be equipped with for instance a Radeon 9200 card. And you even have the chance to use up to R700 GPU family based cards (see above), driver issues aside that is. With built-in graphics functions (and no AGP/PCI) you cannot upgrade the GPU in any way.
Btw, I believe Mac mini G4 was chosen *because* it had already supported Radeon (among other already supported or rather easy to support things), not the other way round.