You will need a GeForce FX 5xxx series card or higher, or a Radeon 9500 or higher. The biggest question though will be whether or not your computer has an appropriate slot for a new video card, and for that, you'll need to open it up and take a look inside.

When you look inside the computer, you should see a large circuit board on the opposite wall of the case from the one you're looking in; this is the motherboard. Lined up along the rear of the computer on the motherboard there will be a number of slots, some with smaller boards sticking out of them. The topmost one will (generally) be your video card.

To put it simplest, your computer most likely has an AGP slot at best; to tell if it has one, look at the topmost slot. An AGP slot will generally be brown and offset about a half inch from the rest of the slots below. Your video card may or may not be occupying it.

If your computer only has white slots, then you're limited to PCI options which are generally slower and more expensive, but there are a few which can run the game.

To get you started, here is a selection of cheap AGP cards that should run the game, and here are some PCI cards.