Where can I locate my pc's video card and how can I upgrade it and not spend alot o cash?

My pc runs 16 colors right now but I have other towers that have 256 color. can I change 'em out?|||Unless you have really ancient video cards or chips, you should be able to get better color on all the video without changing the card.

What operating system are you running [Win3.1, 95, 98, 2K, XP]?