Play the game on lowest settings AND at the lowest resolution the game and your monitor allow. Check the framerate. That will give you some idea, not perfect but close enough, of how high a framerate you might get with a videocard upgrade. What a better videocard will allow you to do is run that framerate at a higher resolution and at higher settings. For example, if your CPU holds you to 45fps average with your current card at all low settings, a better card will still only get 45fps average but maybe at 1080p and high settings rather than 800x600 and low settings.