They support those non-nVidia chipsets through the facilities of the kernel, though, not through anything nVidia has done...
Well, that is assuming you use agpgart. Option "NvAgp" "1" (using NvAgp instead) would require that nVidia's driver work with it. But NvAgp will only be changed with a new driver release, and I wouldn't be surprised if it was quite a while before that happened.
You could always see if 2.5.45 works on your machine, then patch the NVIDIA_kernel tarball with one of the posted patches that has been tested against 2.5.45, reinstall the NVdriver kernel module, and try that. I'm fairly sure that 2.5.45 supports the KT400, though I'm not positive.
Edit: Never mind, it apparently isn't. Just checked drivers/char/agp/agp.c (the file layout has been changed), and the KT400 isn't listed in the initializer list there. What does /proc/pci say the PCI ID is for your northbridge (normally the first device in there)?