I need help with my graphic card.?

I wanna buy a new PCI - Express graphic card, but I don't know if my computer has a slot that works for the PCI Express. I don't want to buy it and not be able to use it. My computer is really old, I use an integrated video card. My computer doesn't have a AGP slot, just three white slots. Will the white slots work for the PCI Express? How do I figure out what the slot is made for? Do both the PCI and PCI express use the white slot?