Are you connecting the monitor directly to your graphics card and not your motherboard?
+1 Sometimes people forget some most basic thing,
If u can change the Display to PCI in bios means u can boot an got to Bios just fine.... after u change and resave the bios do u change the vga cable to the new gpu (not the mobo?)
Can u tell more clearly what do u means sytem not reboot/restart automatically?
Do u have black monitor or system just restarting/turn off after some times??
Can u be more specific? Not being smart ass but more info will turn to more better solution....
Edit: Do u means u not get into OS? I'm have a hard time to understand the situation. Have u tried deafult vga mode (windows) to enter os and install the new driver??