Can you clarify? Are you running a desktop on your droplet on which you are watching videos or are you serving videos from your droplet to watch locally?

All processes you run on your droplet use RAM. You can view details on your available memory (RAM) by running free -m on your droplet and the command top will give you a running list of your processes and what resources they are using. Serving videos will use much less than running a Desktop (which would not be recommended on a 512MB droplet).