fit into the AGP Slot on your Mother Board, or if you have a only PCI slots then you would have to buy a PCI Graphics Card.
Now problem is there are 2 voltages for AGP Card the older AGP Slot is 3v and newer is 1.5v or the other way round. You would need to find out this and id your Motherboard.
Then also should you want to add a AGP Card or PCI you need to know before hand how to switch of your onboard VGA if this is what you have.
Its a case of doing some research before hand.
I suggest you post back with the Motherboard's make & Model or if this is a PAckard Bell, HP or Del etc then the model of the pc.