Here's an update of mine, I upgraded to 32 GB of RAM because I needed it for VMs, but you can see a typical (small) Logic Pro project that is 67 MB in size uses a lot of extra stuff. The project takes about 8 seconds to load off a HDD due to all of the plug-ins, audio files, and midi files that it has to load.
I include a few more columns that I find helpful but the big thing to note is even though my project is only 67 MB in size, I read roughly 500 MB of data from disk in plugins (such as equalizers and instruments), audio samples, automation data, and midi files. My plugins all use the same part of memory but where they might vary in their controls will exist in different parts of RAM (leads to the 8 second copy to memory time). Purgeable memory is memory that is not currently in use and marked for overwrite.
My wired memory is what my RAM uses for all system functions to keep the OS up and running, the App memory is stuff that was kicked off by the system or myself to do stuff, iStat menus runs as a daemon so that wouldn't be something I ran but it's still App memory, the Application itself is something I chose to run at startup so I own it and it adds to memory.
I'm no where near my 32 GB limit, but I've hit the 16 GB limit on some of my larger projects, another reason why I upgraded to 32 GB.
Like I said 8 GB of RAM is usually plenty for basic photoshop because those layers add up. Lightroom is a walk in the park in comparison. If you find yourself getting close then you should upgrade to 16 GB. At this usage, I wouldn't even consider going past 16 GB which is my next logical step past 8 GB.
You don't want to mix RAM sizes say to make 12 GB in my case, although 12 GB will boost performance if you are hitting 8 GB, it will degrade system performance from what you'd get with 16 GB.
Another thing to consider which has a far less impact is using the memory bank channels properly. Never put an odd number of RAM sticks in your computer (the exception being 1 stick of RAM). If you look at where your RAM goes (say you have 4 slots like on the back of an iMac) you will see numbers listed, most of the time the order is something like 1, 3, 2, 4. This is based on dual channel memory, 1 and 2 is where your first two sticks go, 3 and 4 is where the last two sticks go. If you are going for 8 GB of memory you might use 4 GB x2 in slots 1 and 2, but if you had 4 sticks of 2 GB in all 4 slots, you'd get a slightly higher performance boost. For 16 GB you'd use 4 x 4GB and for 32 GB you should use 4 x 8 GB.
RAM should be incremented as 1, 2 ,4 ,8 ,16, 32, 64*, 128* (*Mac Pros or when iMacs can handle it).
You can also increment at 3, 6, 12, 24, 48*, 72*, but you'll run into minor performance issues. (Some computers ship with these numbers so it's not a huge loss and average users probably won't notice).
