PCI Express is becoming the standard for video cards, especially for high end cards or SLI / Crossfire configurations. Single cards are not really pushing the limits of the previous AGP interface, but if you have the choice it would be better to go with PCI-E. The regular PCI interface has not been standard for video cards for years, though you can still find some around. How old is your computer? If you can not find the specs on your video card, your motherboard manual will list what type of slots it has.
GPU = graphics processing unit = graphics card Technically the GPU is the main component of the graphics card (or on-board video), as Alrik described, but the terms are often used interchangeably.
