The pictures here will show how to install a AGP video card.
A PCI video card would be very similar.
Click on pictures to see larger images.
Locate the appropriate slot on your motherboard. AGP or PCI
Remove the metal bracket at the rear of the slot where the video
card will attach to the chassis. To remove it first remove the screw
then the bracket will just come out. In the picture to the right the
correct bracket for the AGP and PCI slot has been removed already.
Set the video card down gently on top of the AGP slot. Make sure
it lines up straight and even with the chassis and the slot. Make
sure the notch lines up with the extrusion on in the AGP slot.
Firmly press straight down on the card it should slide into the
slot. If you are having trouble try pressing down on one end or the
other with more pressure. They can break, so can the motherboard, be
careful.
Then secure the card firmly to the chassis with the retaining
screw.
Attach your monitor to the video card with the cable that came
with your monitor.
Then reboot your PC and the video card should show a video bios
screen first thing. (This is quite fast so your monitor has to be
warmed up to see it.)