No PCI-Express Slot but space on mainboard

I want to upgrade my video card and i was looking at them and there are three basic choices. AGP, PCI, or PCI-Express. I was about to buy a PCI card when i looked on my mainboard and saw that there was a space. There are two PCI slots, then a space with white lettering above it marking it a PCI-Express x16 Card and there is no slot just metal circles and two holes that look right for screws or pegs. Can I install a PCI-E slot and then get a card or not? What’s going on here?