Choosing your video card

What's a video card?!
A video card, also known as a graphics accelerator card, display adapter, or graphics card, is an expansion card whose function is to generate and output images to a display. Some video cards offer added functions, such as video capture, TV tuner adapter, MPEG-2 and MPEG-4 decoding, FireWire, light pen, TV output, or the ability to connect multiple monitors.
A misconception regarding high end video cards is that they are strictly used for video games. High end video cards have a much broader range of capability; for example, they play a very important role for graphic designers and 3D animators, who tend to require optimum displays as well as faster rendering.

Things to consider before buying a video card
Motherboard interface
Graphics processing unit (GPU)
Video memory
Outputs
Power demand

I will explain better each of this aspects.
Motherboard interface
The motherboard interface is what type of socket does your motherboard support. Ex: Agp, PCI express. Depending on what motherboard you have you need to check which type it supports. If not both.

Graphics processing unit (GPU)
what's a GPU?!
A GPU is a dedicated processor optimized for accelerating graphics. The processor is designed specifically to perform floating-point calculations which are fundamental to 3D graphics rendering. The main attributes of the GPU are the core clock frequency, which typically ranges from 250 to 850 MHz, and the number of pipelines (vertex and fragment shaders), which translate a 3D image characterized by vertices and lines into a 2D image formed by pixels.
In other words it's the graphics processor, highest the better. Note that If you do not plan to use the computer for gaming it doesn't really matter.

Video memory
The memory capacity of most modern video cards range from 128 MB to 4.0 GB, though very few cards actually go over 1.0 GB. Since video memory needs to be accessed by the GPU and the display circuitry. This means that when you are playing games most of the ram is consumed by the video memory and not your physical memory. (note that's only for graphics)

Outputs
The most common connection systems between the video card and the computer display are:
Video Graphics Array (VGA), Digital Visual Interface (DVI). This tells you what type of output you must have on your monitor.

Power demand
As the processing power of video cards has increased, so has their demand for electrical power. Present fast video cards tend to consume a great deal of power. While CPU and power supply makers have recently moved toward higher efficiency, power demands of GPUs have continued to rise, so the video card may be the biggest electricity user in a computer.Although power supplies are increasing their power too, the bottleneck is due to the PCI-Express connection, which is limited to supplying 75 W. Nowadays, video cards with a power consumption over 75 watts usually include a combination of six pin (75W) or eight pin (150W) sockets that connect directly to the power supply to supplement power. This is important so you can then choose your Power supply.

Also most of today's motherboards already have on-board video cards. So if you are not using your computer for high end gaming or image editing. You can same some money by not buying a video card.