i had exactly the same problem as you. suse 8.1, newest nvidia driver, geforce 2 mx 400. When starting x, ascii-garbled screen and lockup with just ctrl-alt-del working. I solved it first by adding
Option "NvAGP" "0"
to the /etc/X11/XF86Config file in the device-section, directly below the "driver"-line. Then it worked, but without agp support. Setting the Option to "1" (See /usr/share/doc/packages/nv_glx/readme) should have enabled nvidias agp support, but didnt work (x started, but still no agp). setting it to "3" (enabling the kernel-modulized agp support via agpgart) reproduced the crash. that changed when i entered the BIOS and changed the agp rate to 1. Now it works!! With agp support. You can check if agp is working by typing: cat /proc/driver/nvidia/agp/status when x is running. Good luck!
But my problem still is: Every opengl game has short "locks" (about 0.3 sec) in regular intervals, although e.g. tuxracer runs with 70 fps. Does anyone know how to fix that?