You might be able to back down to 4x by installing the NVIDIA_kernel from source -- specifically, from .tar.gz.
Unpack the .tar.gz archive, cd into the new directory, and edit os-registry.c. At the bottom, in the nv_parms array, remove the two underscores from the "__ReqAGPRate" parameter, so it looks like "ReqAGPRate" instead. Then save & exit, and run a make in that directory. Then su to root, and run make install (make install has to be run without X running, so that it can remove the old kernel module from memory).
Then edit your /etc/modules (or whichever file it is that Mandrake uses to automatically load modules) to add an NVreg_ReqAGPRate=4 to the nvidia module command line. You could also do it with an options nvidia NVreg_ReqAGPRate=4 line in modules.conf if that's how this module is being loaded.
Then start X up, and see if that helps.