There are a lot of video cards available on the market, but when it comes to selecting one, you're basically limited to two chipset manufacturers: NVIDIA and ATI. 

When looking for a video card, you need to know the type of interface the card connects to your motherboard with. An older style would be something like AGP 4X/8X, while a newer style would be something like PCI Express.

You also want to make sure the video card you choose will actually fit in your computer, because there is no one default size.

It's also a good idea to look at the Memory Size of your current card as well as the speed (Effective Memory Clock), Memory Interface and what the card supports (DirectX, OpenGL, etc.).

Take note of which Ports the video card has. This will help you determine whether you need a card with HDMI, DVI or VGA capabilities.

TomsHardware.com has a lot of information regarding video card breakdowns and specifications. Check this website for video card information.