Without a video card, you would be reduced to viewing a black screen since there wouldn't be any way for the computer to display what it is doing. With the current OS's being more graphic intensive, you need a better (and faster) video card to keep up. Without a video card to take on the display processing, your computer would run at half speed (if you are lucky). The more graphically intense the application, the more it will use your video card. That is part of the reason why games have minimum requirements, and why even applications like PhotoShop perform better with better video cards (even if the rest of the computer is identical to another).
I saw a performance boost when I upgraded the graphic card in my (formerly my) G4 tower. I went from the Rage 128 (16MB) that it shipped with to the ATI Radeon Mac edition (with 32MB of DDR memory).
That help you any better???