Scaling questions


My app is currently running on a single DO instance.

At peak times its hitting 500mb ram atm and I only see that number growing. My instance has 1gb ram. Peak cpu usage is 30 percent.
When do I have to start worrying about upgrading my server or adding more servers?

Also, horizontal vs vertical. Vertical is easier but horizontal gives me more control and allows me to downgrade with east if I want. What do people recommend?