I think you're seeing three different issues here.
First, you saw a problem where X took a high % of CPU and you just had a black screen. I'm not sure what to try with that one, other than maybe taking a look at some of the other Mandrake 9.1 threads, or the other KT400 threads (though I think that driver 4363 is supposed to fix the KT400 issues... well, as long as you're using Option "NvAGP" "1" in your Device section, that is).
The second problem I'm not sure about, since "no screens found" is not the actual error (the actual error either was printed up higher on the screen, or is recorded in the X log), but I'm pretty sure it's that you didn't have the "nvidia" kernel module loaded. Try /sbin/modprobe nvidia as root before doing startx as your normal user, and you'll probably get the same blank screen problem. If so, add alias /dev/nvidia* nvidia to /etc/modules.conf, and run /sbin/depmod -a as root, to make the module autoload when it's needed (with Mandrake, the usual cause of not getting that alias put in there by the driver is that you booted to failsafe mode when installing).
The third thing is that you were still loading nVidia's "glx" extension (libglx.so), but you weren't using nVidia's X driver. The "glx" extension needs the Driver "nvidia" line to be in use before it'll work right.
Fix the modules.conf, and revert to Driver "nvidia", and you should be back to the blank screen. Fix that, and you'll get a working driver (well.... hopefully).