but I have a third unknown device

Probably ATI Rage Theater chip.

Thanks Serpent Rider, now that you mention that, that rings a bell. I'm surprised the drivers (10.2 from the AMD site) don't include the driver for that.
I installed UT99 for an additional test, and it seems to work fine. Perhaps I'm just running into quirky 3dmark issues, although quite surprised 03 won't run as all the hardware is pretty close to that period.
J

ATI would sometimes put the Rage Theater driver in a separate download. Sometimes not even have it listed with a particular Catalyst release. It is annoying indeed.

A further observation, UT99 works flawlessly (except the lack of resolution options, however UT03 is a garbled mess. The audio works, but the graphics are a mix between wireframe and random patches of shading.

Hey that card has the AGP to PCIe bridge chip. That is something that tends to cause problems with various motherboard AGP implementations.

You might want to search the forum and see if anyone else ran into issues with KT600 and that chip. It's called Rialto.

Hi Swaaye, I hadn't thought of that. Those bridge chips were pretty common towards the end of the AGP era - cards were developed for the PCI-e bus, then 'retrofitted' with the AGP interface and bridge chip to make them work. I had a look around, didn't find anything specifically on this forum, however I found a few mentions around the web about compatibility issues with VIA chipsets overall. I think my best bet is to pick up another AGP card that pre-dates the arrival of PCI-e then I can avoid this problem. Unfortunately the nVidia FX series and Radeon 9xxx series seem quite collectible currently 🙁

Edit:
As mentioned, this PC in its previous incarnation was running a Radeon X850 Pro AGP on Windows 98. I ran this PC for many years without issue. I assume that the Radeon X850 Pro AGP also uses a bridge chip, I'm wondering if Windows 98 is more tolerant of these chips. With that in mind, I'm tempted to put Windows 98 back on here and see if these issues go away.

Stirring the brain here... You may want to try a wide range of Catalyst releases. I ran an X800XL with the bridge chip for awhile and I remember ATI had poor QA testing for their bridged cards. In fact I remember there was some amount of releases with broken D3D. I had the card on a K8M800 motherboard.

I'm not sure what the oldest driver supporting the card is but that would be a place to start. Drivers prior to 5.9 have truform support by the way. But 5.8 seems to have very poor truform performance so probably avoid that one.

AGP-Rialto cards also had special driver releases.