If the system opens up properly after putting in new or extra RAM, there're nothing more to do except check in Control Panel>System to check that the figure is correct.
You can also bring up a Run box by right clicking on your Windows square and R at the same time. In the box, type msconfig and hit the Enter key.
Under the Boot heading, you can make sure all the cores of your CPU are in use at boottime and that the maximumum RAM is also in use.