I can't help but I can confirm the problem.
I've had very bad lockup problem with both 4XXX drivers. No log files record the lockup, but it happens very often under SMP kernel (usually every day, X crashes once an hour).
Dual 2.2 Xeons
Dell Precision 530
Quadro 4 750
2.4.18-3smp (up fixes the problem)
Using 3123 drivers fixes the problem. (Uptime of two weeks without a crash/lockup, either X or system, no 3D though.....)
Whatever the NVidia guys did with the 4XXX drivers, they broke the SMP compatibility.
I've tried diabling AGP and all the power managment stuff, but the system won't stabilize under 4XXX drivers.
NVidia? Are you listening? People have been reporting SMP lockups since 4191 was released........ Everything works fine until the system locks up.