Your ATI is onboard but is a very high spec. You can add any card you want as long as it is compatible with 1 of the expansion slots on your motherboard (AGP, PCI Express, PCI Express 16 etc). If you use a slot card, fit it first, put your screen on the new card, boot up and go into the BIOS. Find the display and change it from onboard to XXXXX where XXXXX is the slot you have installed in. Exit and save the changes and let it boot. Your graphics will be basic until you install the drivers that came with the new card.