True. But remember to buy something that, like Drinkey said, is compatible to your OS driver (or the ones you will need to install) and also compatible with your MOBO. Even more so, a graphics card is probably one of the easiest things to replace in a computer. However... Too many times does someone go out and buy a kick ass graphics card that exceeds their replaced relic only to be disappointed in actual use. Why? Because most of the time the monitor you have is old, the MOBO won't support the most current features the graphics card comes with (Direct X 11, HD, etc etc), and even your OS might not be the latest in the batch of Windows or MAC OS.
So, just head on over to a Fry's or a Microcenter if you are lucky, and ask someone. OR the best and easiest bet is to look up your computer specs, find out what MOBO (mother board) you have, and see what is compatible. Chances are if it is an older system, there is plenty information out there.