It is ironic that in an age when electronics manufacturers have consistently reduced the sizes of components, computer graphics cards seem to be getting bigger all the time. On the positive side, the increase in size comes with incredibly boosted performance, and modern cards can do things that were unthinkable just a few short years ago.
Your choice of computer graphics cards will be based on what you will be using your computer for. If your applications will not be very graphics intensive, then a pretty basic card will be absolutely fine.
If you will be using your computer to play the latest games, then you will get a lot more pleasure from a state of the art card that can deliver excellent definition, and is fast enough to run your games without any lagging. Similarly, if you are going to work with applications like Photoshop, or you will use CAD software, it pays to invest a bit more and buy a good quality graphics card.
The graphics card will take a lot of the processing away from the CPU, using its own on-card memory to do most of the manipulation. This takes some of the pressure off the CPU, leading to better overall performance.
The price of cards can very from being very modest to being one of the most expensive components in your computer, so you will need to know how much you can afford to spend before shopping around for computer graphics cards.
You also need to make sure that there is enough room in your box to take your chosen card. If you already have lots of cards added, you could run into problems, as some of the newer graphics cards actually take up two slots.
Finally, you need to make sure that the display connection on your card will work with the monitor you have. Most new cards do not have a VGA port, for example, although you can buy an adapter if you want to use a VGA screen.