Similar problem here..
I am also using RH9 with an FX5600 and I too get lockups. Mine are somewhat random though and in most cases T have been able to telnet in and shut it down in an orderly fashion.
A much more annoying problem is that X just dies in the middle of anything and when it respawns I am presented with a logon screen. Obviously all work is lost.
I have read all the postings and it would seem that the most likely cause is that the driver is buggy. Has anyone brought this to Nvidia's attention? Do they read these posts?