Again the graphics card is another very important piece of hardware inside a gaming pc, as i said before many motherboards will include an onboard graphics card, this means the motherboard has the capabilities of producing 3d graphics, however, installing a seperate GPU takes the strain away from the motherboard allowing it to concentrate on other aspects of its responsibilities. Graphics cards can have an extorsionate price range and you may find yourself baffled whilst searching for the right one. The main thing to concider is which card is right for you. nVidia or ATI are the two to choose from, even if the card comes stamped with another name its these two chips that are ultimatly responsiable for the GPU's performance, take a look at the nVidia vs ATI section for more details.
