How to choose a perfect graphics card

First, you should decide on what will you do with your graphics card
so as not to waste your money.For business, a graphics card like ATI
Radeon HD 4870 is a loss!
Now,I'll illustrate how to get the best graphics card for gaming.There
are some things you should care about.
1.The GPU speed:
GPU is a processor designed for graphic,so the
speed is important to make your games run smoothly.So, it's advisable
to get a card with 550Mhz or faster core clock. Be careful, the core clock may be misleading as there are more important factors that should be considered, the some of  them is the shader clock for nVidia cards and processing streams for ATI ones. You may  buy a card with 1200 Mhz or
faster shader clock(for nVidia cards only) or 800 or more processing streams for ATI cards.
2.Memory:
 For running games faster,the graphics card should have 512 MB memory
at least.Also, the memory interface is important and should be 256bit or
more.The video RAM is important for better shared quality and details for most new games.
3.The doubled performance:
Now you can get a doubled performance with two ways.
The first is to put two graphic cards or more on the motherboard
to work together. This technology is called Crossfire X for ATI and
SLI for NVIDIA.
The second is to buy a dual core graphic card.You will find "X2"
in the end of the name of dual core graphic cards.
Here,before using any of them you should take a great care of your power supply and make sure that it can run these cards or you will have to buy a new one.
4.Other technologies:
The graphic card must support the following:
-Model shader 4.1 or later.
-PCIe 2.0
-Direct X10.1 or later.
-OpenGL 3 or later.

5.How to understand the code name of graphics cards:
nVidia and ATI use a number (code) for their products.
for each series of graphics cards you may notice the name of it (like ATI HD 4770) contains for numbers.
the the first one (from left) is the series number and the more important is the second  one.
for cards coded  x4xx  deliver the essential performance like playing hd movies and a few old games.
for cards coded   x6xx  are the very good for users who want to play games at midum  settings and note paying very much.
for cards coded  x8xx is the most powerful and could deliver high performance for most new and upcoming games.


Also, see a comparison of 6 powerful ATI HD 3870 .