It is the integrant device in the computer system, its function is to transmit video data from CPU to the monitor that we can understand. It is the most important pipeline that we get the information from the computer, so the display card and the monitor are one of the important part for the computer.
The 3D display cards, which involve a large ram, are for the professional drawing and playing game, etc, such as TNT and Geforce. So, if you are not using in this way, no need to buy too powerful one, because one powerful display card is seem expensive. Otherwise, you can consider to buy a higher quality one if you want to play 3D games.
The sound card is the similar kind of the display card, it is to transmit the sound wave message into the voice through the speakers that we can hear. If one computer have not been installed a sound card, the computer cannot be able to process the sound wave message, than we can't listen any music.
Whether the timbre is good or not, is based on the sound card and the speakers, that means if you buy high quality speakers but the sound card cannot support, it is also useless. So you need to consider how to balance.