It's time to break out the tape measure. Almost all modern desktops can fit at least some kind of external graphics card, as long as they have at least one PCIe slot on the motherboard. But before buying a new card, make sure your case can fit it, both in length (measured in millimeters) and in slots (the number of eternal notches at the rear of the case, not physical PCIe connections) it needs. Most mid-range cards use two slots, but more powerful ones can use three or even four. You also need to make sure that your computer's power supply can handle the additional watts drawn by the new GPU, and that it has the correct power connections to supply electricity to it. If not, you can always upgrade your power supply, too.