sorry, kt400 stinks
You did insmod nvidia.ko and it failed, I see.
If you had agpgart in the kernel, you have about one option. Take it out, let nvidia.ko do it's own agp!
I didn't try that. I just yanked out the kt400 mobo and stuck in an nforce2 mboard because my kt400 was only agp4 and I was stuck where you are, too.