i updated my sis onboard graphics to a agp card.its quite straightforward and should be the same for most systems.
first asuming that this is windows xp,go to controlpanel/system/devicemanager and look for display adapter or a reference to the onboard graphics.right click it and select uninstall and then shut down the computer.
open the case and look for the agp slot(usually brown in colour and above the white pci slots)slot in the new card and turn on the computer.
on starting enter the bios by tapping F1 F2 F5 delete etc(it will say on the screen)and check under advanced chipset for onboard or shared graphics and if not disabled,set as.also check the agp is set to x4 or x8 as the specs for the new card and agp app size is set to about 64 or 128(not that important so long as its set to 16 or over)exit the bios saving changes.
when windows starts up it should now detect the new card and prompt to install the drivers.my advice is to click cancel(twice if secondary display is found)an instead of installing the drivers from the cd get the latest ones from ati or nvidia as the ones on the cd will be out of date.click here click here