The terms agp and pci-e relate to the slot you have on your motherboard for the graphics card.
If the motherboard (or PC) is more than about 2 years old it is probably an AGP slot, if it is a newer motherboard or PC then it is probably a PCI-E slot.
A motherboard can EITHER have an AGP slot or a PCI-E slot (not both) so you need to buy the right graphics card for your motherboard.
Obviously whatever your existing card is (agp or pci-e) you need to buy the same type of card.
Note that an AGP card will not fit in a PCI-E slot, and a PCI-E card will not fit in an AGP slot.