I found a picture that shows the difference based on the "key" pattern of the card. Unfortunately, you'll have to open your computer and remove your videocard to check which kind you have. Careful removing the card, if you bend it sideways or apply excess pressure to either the card or the components on it, you can break it. Click
HERE for that picture. You want to look for an AGP or a 16X PCI-Express. The Third type is for other cards, like soundcards and network cards.