Exactly how you handle this depends on the model of Vectra you use. Some of the newer ones will likely auto-detect the new video card and disable the onboard.
The way you do this "manually" is either to go in the bios and disable the onboard video controller or (for older systems) set a jumper on the motherboard to disable the controller. You may need to check your documentation for this.
You can install a video crd in the PCI or AGP slots on the motherboard,it will not override the the onboard video.
Boot the system and enter the Bios,then locate the CMOS configuration to your onboard video,select to disable. Exit and Save
Reboot computer and configure the video card in the system using the software that accompanies the video card,you will also want to check the manufacturers web site for updated drivers.
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Video Cards