You’re also going to need to determine what type of RAM your computer is capable of using. Just hit up Google with your model number and you should find what you’re after. If you’re unsure what motherboard you have, you can again turn to Speccy, where the Motherboard category shows you the information you need.
You’ll need to check the documentation for your motherboard or PC for the details. The second part of the equation (how much your motherboard can handle) depends entirely on the manufacturer, though most modern computers will support at least 8 GB, and more likely 16 GB or more.
The Home Basic edition can handle up to 8 GB, Home Premium up to 16 GB, and Professional up to 192 GB. On Windows 7, things are a bit different.
And moving beyond 16 GB is going to be a smaller boost still. Moving from 8 GB to 16 GB still shows some good gains in performance, but not as much. Moving from 4 GB to 8 GB of RAM is likely to make a huge difference. That said, the law of diminishing returns applies.