installed windows, graphics are terrible

I have an old compaq deskpro that had windows 95. I formatted the C drive and installed 98. The graphics are horrible. When I go to “display” in the control panel, under “color” it only lists two options: 2 color and 16 color. Under screen, it says “640×480” and the slider will not move to a higher option such as 600×800. I installed windows 98 on another old computer and the same thing happened. How do I fix this?