Question about how to buy right video card for my desktop?

I have a compaq computer, kinda old. I just had to go buy an ethernet card for it cuz it didnt have one and the only way to connect my comcast modem was with ethernet. Anyways I need to buy a video card cuz when I try to watch a video , it is very choppy and someone told me I need a new video card. How do I know what is the right one to buy?|||Totally depends on your computer. Need details. More ram might help too/or instead.