The particular games may have adapted themselves to suit the characteristics that were available when using the 'old' graphics driver. You could try running the game setup program options again (this'll depend on the game so read the games manual/website about how to do this if you're unsure) to let the game redetect your graphics capabilities. Also if your monitor is not correctly installed and configured eg the correct model and resolutioon / refresh rate, it'd be worth installing the correct latest drivers/inf file for that.
