I'm almost certain the intel 82845g is an on-board graphics chipset and so you will be adding the card. You will probabbly need to enter the bios in system startup to disable the onboard graphics once you have installed the new card
One word of advice though, be careful about which graphics card you get. In my experience a lot of computers that have on board graphics (From the likes of Compaq, HP etc) do not have an external AGP interface (ie an AGP slot), the only AGP interface being that with the onboard chipset. If this is the case you will need to ensure that you buy a PCI card. Unfortunatly this will limit the range availiable to you.