When should I upgrade the RAM in my new iMac 5K?

[johnnyXcrane]

As far as I know the Activity Monitor anyway always say the RAM is full because it cache all, thats right?
So how can I know when it will be useful to upgrade my 8GB RAM?

 

[GGJstudios]

As far as I know the Activity Monitor anyway always say the RAM is full because it cache all, thats right?
So how can I know when it will be useful to upgrade my 8GB RAM?

It is quite normal for all of your memory to be in use by OS X. It does not mean that you are running out of memory or that it is maxed out. OS X will manage all available memory, making it available to apps on an as-needed basis. Refer to the following Apple support article for more information on how to understand your Activity Monitor readings.

OS X Mavericks: About Activity Monitor​

The combination of Free, Wired, Active, Inactive & Used memory statistics in previous versions of Activity Monitor have been replaced in Mavericks with an easy to read "Memory Pressure" graph.
Memory pressure is indicated by color:

• Green – RAM memory resources are available.
• Amber – RAM memory resources are being tasked.
• Red – RAM memory resources are depleted and OS X is using the drive for memory.