As all of you might know already, Memory or simply Randam Access Memory(RAM) is used to retrieve the most necessary data from Hard disk quickly so that the CPU can access and read them quickly.
Before any software launches, it retrieves necessary data to run them. All program runs quickly if RAMs are installed. Retrieving data from Hard disk is still not fast enough.
Playing back movies and playing games, require a constant read and write frequently.
In order to experience the smoothness, you need a fast, quality RAM. The amount of ram is not that important if your ram is super fast.
Unfortunately, the price is still expensive compared to the performance you will gain by installing them. Windows operating systems do require a lot of rams. At least 2Gb today in order to run smoothly. Other operating systems such as linux do not need that far.
RAMs do not speed up the system but the loading time of any applications will be reduced.