Generally discrete graphics cards are needed only for gaming - which requires 3d processing by the card. 2d graphics can normally be handled by integrated graphics - if you have one of the more recent versions. The more recent ones can also handle HD. But if you have an older model integrated graphics, or a large resolution monitor, at some point you may need to add a small discrete graphics cars. You can find more info about upgrading non-gaming video cards at:
http://www.upgradevideocards.com/nongaming.html
Video cards are being adapted to aid the CPU in some types of processing. But you probably do not have to worry about that yet. Maya and Photoshop CS5 have some features optimized to take accelertate processing by using the CUDA feature on nVidia cards. Premiere CS5 and some other video editing software can also take advanage of and use the video card to accelerate processing.