I'm having what seems to be exactly the same problem as in the following thread: nvidia 6200 and ATI RS480/482 chipset incompatibilities
In that thread a bug (227803) in the nvidia driver was identified and later fixed, but only for the specific card it seems as my ubuntu6.10 with a 7300GS card still locks up hard and the only solution is to reboot.
If I use the nv driver the pc is rock solid (not a single crash no matter what I do to it).
I've attached the nvidia-bug-report.log, but since I can't generate it when the problem has occurred I guess it's of limited help.
MoBo: MSI RS482M4 (Latest BIOS v1.2)
Gfx: XFX 7300GS
Driver: 97.55 9755