Install the agp card , and set bios to onboard agp , then hookup the video cable from monitor to the new agp card only .
The monitor should be switched on as you startup the computer because these things check for power status of connected devices .
Uninstall all video drivers , then reboot into safe mode , here in device manager , you need to remove all video adapters .
This is one thing many technicians forget , and it will cause conflicts when the computer is started and running in normal mode operation .
Back in normal mode , if bios was set to pnp , it should automatically detect device and install appropriate drivers for it , if not , you have to do it manualy by add device in control panel , when done reboot .
Please also update to the latest usb drivers for your motherboard prior to video driver installation .
Failing this , you can consider you motherboard has a fault , and may require a bios update .
Flashing the bios is not to complicated , but .....
The utilities do have safe guards , that is , they must backup original bios , then write new bios update , if no error , your ok .
If error , get the utility to replace the new bios with the original backup bios , this returns the system back to original setup .
Also note , when motherboards get on , 3-5 years , sometimes the pins become oxidized , this can happen to the bios chipset , which sits in most cases in a small socket .
Note : do not attempt this with power connected to pc ....
It should be carefully lifted with extreme caution with a small flat plastic knife , slowly and evenly on all sides to avoid cracking the chip , then simply reinsert the chip back into position , this may also cure other common problems .
Hope this technical knowhow helps you out .