FAQ - How do you know it's time for a memory upgrade?
There are several signs indicating it may be time to upgrade your memory. If you see your mouse pointer turn into an hourglass for significant periods of time, if you hear your hard drive working, or if your computer seems to work more slowly than you expect, the reason is probably insufficient memory. When the memory is full, your system transfers data to the hard drive. This is called swapping. Since the hard drive is considerably slower than DRAM, your system seems slower altogether.