Yeah, maybe. PCI should have a way to probe for that kind of configuration information, regardless of the device, though.
The 8X AGP might indeed have something to do with it, but I don't know enough about it to suggest anything, unfortunately.
Yeah, I don't think the problem is the video card, it's probably the KT400. But you're welcome to try it, of course.
You said that later 2.5 kernels do support that chipset? Or didn't you get that far? There are patches somewhere to get NVIDIA_kernel to compile against 2.5 Linux kernels, so maybe trying one of those would help?
If the KT400 is supported in recent 2.5's, then you might try looking back through the changelogs to see when it was added, then taking a diff of pci-ids.h and the drivers/char/agp directory (diff between the version right before support was added, and the version that it first worked with, obviously) to see what got changed. See if you can maybe hack those changes into 2.4.19, or something...