One of the simplest ways of determining whether you need to purchase additional memory for your iMac is to find out how much memory is free when you are running your normal applications. If you find that you have very little memory free, you will know that your iMac is running out of breathing room and that you may experience increased performance by upgrading your RAM. Use the Activity Monitor on your iMac to determine how much memory is being used and how much is left.
Click "Go," then click "Utilities."
Click "Activity Monitor."
Click the "System Memory" button. The amount of memory in use is shown next to "Used," and the amount remaining is shown next to "Free."
Click on any process, and click the "Quit Process" button if you need to close an application immediately to reclaim memory.
The Activity Monitor can also be used to view current CPU activity on your iMac as well as the amount of hard disk space that is being used for virtual memory.