Graphics via motherboard bad need to install card

Hello. I need information in general, nothing specific, because I don't have that info. What I have: a friend's computer running MS Windows XP SP2 has a port on the motherboard, to which the monitor is connected. That port no longer works, the monitor is dark. Switching out the monitor with a known working monitor yields the same result, nothing on the monitor. So the theory is the port on the motherboard is bad. If I install a PCI graphics card in this machine, how will I tell the computer to use the new card and not the old port? When installed, with the system just start using the new card? Thanks, and sorry for the lack of specifics, hope you can help with what I've given.