A PCI Express card will not fit into a normal PCI slot. They are two different things. And yes, it is confusing! PCI Express cards are generally faster than AGP cards, as PCI Express is newer, and supports faster speeds. PCI Express is supposed to be taking over standard AGP cards and slots. It just depends on what you want from your graphics card and if you have a PCI Express slot available on your motherboard.
You'll have to find your motherboard model name or look on the manuals which should be on your computer to find out if you have a spare PCI Express Slot.
Hope that made sense and helped! It is correct to the best of my knowledge!