The key components and features of video cards are:
The GPU (graphics processing unit) or VPU (visual processing unit) is a digital circuit integrated into the video card body and is used primary for 3D applications. The GPU processes information and sends it to and from the Video memory.
The Video memory has two main features, the memory clock rate and the Bandwidth. I've taken the following table from Wikipedia:
Memory clock rate(MHz) Bandwidth (GB/s)
DDR1 66 - 950 1.2 - 3.04
DDR2 533 - 1000 8.5 - 16G
DDR3 700 - 2400 5.6 - 156.6
GDDR4 2000 - 3600 128 - 200
GDDR5 900 - 5700 130 - 230
This table displays how the video memory clock rate and bandwidth evolved from DDR1 to GDDR5.
Modern video cards have sufficient memory to be used by the GPU. Usually a buyer first looks at the memory quantity, but remember that the most important component of a video card is the GPU. The GPU is responsible for the video card's 3D performance (used mainly in gaming). There are many types of the same GPU, this is why you should also look at the frequency. Choose between Cuda cores (nVIDIA) and stream processor (AMD) or between Crossfire (AMD) and Sli (AMD) technology.
The memory Bus - the communication channel between the GPU and the video memory. This is used to send information between these two hardware components
The output connectors are the ports used by external peripherals (such as a monitor) to connect to the video card. There are many types of connectors like HDMI, DVI or VGA and there are video cards that offer multiple display connectors.
The cooling system, which is commonly composed of the heating fan and a radiator, is another important component because the GPU must remain in the temperature parameters all the time.
The digital and analog resolution - this component specify the distinct pixels in each dimension that can be displayed. It is characterised by the number of pixels per width × height.
Direct X and OpenGL compatibility - these are standard APIs used by video drivers to specify 2-D and 3-D image processing. These two types of APIs are composed of elements that specify how the video component is processed. The next tabel from http://www.cprogramming.com/tutorial/openglvsdirectx.html will display the differences between OpenGL and Direct X:
|Multiple Operating Systems||Yes||No|
|Development||Multiple member Board||Microsoft|
|Hardware independent Z-buffers||Yes||No|
|Depth of field||Yes||Yes|
|Parametric curves and surfaces||Yes||No|
|Cache geometry||Display Lists||Vertex Buffers|
|System emulation||Hardware not present||Let app determine|
|Source Code||Sample||SDK Implementation|
Maybe the least important factor is the physical size but of course you should check it out just in case the video card doesn't fit your computer configuration. The parameters of the physical size are width, height and thickness.
I've pointed out all the main components and features that make up a video card, leave a comment if you have any questions or if you have something more to add. Stay tuned for more to come, meanwhile enjoy your day.