I have the same problem, with nearly an identical setup. I have a GForce 2MX card and a Riva Vanta both displaying at 1280x1024. If I shutdown/reboot
without shutting down X the machine will hang. The machine itself is an Athlon Tbird with a VIA chipset.
I suspect this is a problem with the NVdriver hanging when it tries to shut down/unload. The only work around that I know of is to shut down X before rebooting (e.g. - switch to runlevel 3, or whatever will get rid of xdm/gdm/kdm for you on machine).