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The GPU as we know it, is a tiny chip that is strapped to the PCB, or in simple terms, the "circuit board". It is this PCB which we plug into the motherboard. The relation between the CPU and the motherboard is similar to the relation between the GPU and the PCB.
The GPU is made by the 2 main players in the graphics market: AMD (formerly ATI) and nVidia. They manufacture the GPUs and give them to third party manufacturers, who in turn, design their own custom mainboard for the GPU, add their own custom cooler and sometimes overclock the GPU before packaging and selling.
Now these companies vary in terms of performance, build quality and after sales service. In Calcutta, ASUS has a bad reputation because of poor after sales service. You can go for any of the companies you can find, as long as you make sure of the quality of after sales service you will receive.
One small point you should remember: PALIT, EVGA and ZOTAC make nVidia cards, while HIS, XFX, PowerColor and Sapphire make AMD Radeon cards. MSI and ASUS make cards for both nVidia and AMD.
Cheers
