All the information you are looking for is at the bottom of the activity monitor.
1) Check the memory pressure. As long as it's green, your machine is good.
2) Check the Compressed RAM. When Mavericks running out of RAM, it will start to compress the memory. As you can see in my screen short. I only have 32G of RAM, but 8G of RAM is compressed, and my machine is actually running with 48G virtual memory (this mean the apps ask for 48G of RAM). My Mac still no need to use swap. If the compression really close to the limit, the memory pressure will turn yellow.
3) Check the Swap used. If the OS can't fix the problem by compressing memory,
the memory pressure will turn red, and start to use HDD as virtual memory. the swap usage should record all swap in and out activity. e.g. 100mb swap in and 100MB swap out, the swap used will display 200MB. So, if the system use that 100MB virtual memory by 20 times, you will see 4G in swap used. It doesn't mean that your HDD virtual memory is now 4G in size, or the system demand another 4G of RAM.
Therefore, unless your "swap used" keep increasing, there is no need for you to install more RAM. However, if the system keep using memory compression. That means you are really close to the limit, may be it's time to upgrade. Also, memory compression will cause little performance hit as well. Not 100% ideal.
P.S. Some user report that their Mac shows a stable 2xxMB in swap used with no memory pressure at all. I don't have this problem, and don't know the reason yet. But again, as long as the swap used is not increasing, your Mac have enough memory to use.
