Depends on the operating system you are currently using, if there are
drivers available for it the video card and also whether your current card
or onboard video is compatible.
Many onboard video systems turn off as soon as there is another video card
installed in a PCI or other slot on the motherboard. Others are controlled
in the bios.
Without knowing a lot more information (current video setup and
make/manufacturer, motherboard make and manufacturer and old video card make
and manufacturer no one here can even make a guess.
Some may tell you to try it to see if it works. I won't do that without
further information since you may render the onboard video unusable and
until the drivers are installed for the old card, you won't have very good
monitor output (if at all.)