Well, I was having the same problems with my new MB (ECS L7S7A2 -- SiS 746 chipset). It would try to startx, fail, and seconds later bash would crash.
If you look in /var/log/messages and search for nvidia, you may have a message simular to:
Jun 9 18:16:18 dhcppc2 kernel: 0: nvidia: Can't find an IRQ for your NVIDIA card!
Jun 9 18:16:18 dhcppc2 kernel: 0: nvidia: Please check your BIOS settings.
Turns out I just need to set Assign IRQ to PCI VGA to true (even though I am using an AGP card).
Hope this helps!