The amount of Ram that you need is determined by several things that can limit what you actually want or need.
a. Operating System.
b. Motherboard limitation.
We all know that a Windows 32 bit operating system will only see and use less then 4gb of ram so if you have that 32 bit OS then you have no choice with the amount of Ram unless you have 1gb or 2gb and you want to go to 4gb.
A 64 bit operating system will allow , see and use all the Ram that you need with the exception of a server setup that can need hundreds of gigabytes and a normal end user will most likely have a motherboard with a 64gb limit.
The Motherboard can be restrictive especially if you have an older Motherboard the Ram limitation can be as low as 8gb. However the newer boards will have a limit of 16gb, 32gb and 64gb depending on the model that you choose.
How much Ram do you need?
No not this Ram.
Seriously now what are you using your computer for that needs certain amounts of Ram? There are programs that are Ram hungry and will chew through big amounts and be wanting more.
1. 3D Graphic Modeling
2. Photo and Video editing
3. Video rendering
Working with video and pictures will tend to require large amounts of Ram so it's no surprise that when it comes to gaming that you will also need some extra Ram and you can end up needing it in both the system operation and the video card itself. Video cards started out with a small amount of video ram and now they are currently running amounts that can rival what's installed on the motherboard.
Games did not use to ask for large amounts of Ram to be required to play the game but that is slowly changing.
Just last year I sat in front of my computer playing BF3 and had the resource monitor open showing CPU usage and then switched to memory usage and noticed that of the 16gb of installed Ram I was utilizing 5.5gb of it. Not a lot when you consider that there were some things running in the background while the game was running.
That is about to change with the new release of Call of Duty "Ghosts" that shows a system Memory requirement of 6gb.
This is an interesting option if you have a computer with the ram maxed out or close to it, but it's also something to consider if your building a new computer or upgrading an older one. What if you want to have a Ram disk?
A Ram disk offers insanely high read /write speeds bit is not an ideal solution as it does use Ram which being volatile memory will be wiped clean once the computer shuts down or loses power so a back up software program would be needed to save what's on the Ram disk.
You will need to decide what the computer is to be used for so that you can adjust for the amount of Ram needed. Doing work related video and photo editing will require at least 16gb and the more the better with 32gb being a good working amount.
With gaming it will be a lot less that would be needed and with the new games coming out the normal 8gb that was recommended in the recent past will now be upped to 16gb. Naturally if there are budget restraints and you have to go with 8gb then it should be ok but not optimal.
In the event that a Ram Disk interests you then you will need a good amount of Ram so that you have a decent sized Ram Disk and still have enough Ram for running system operations , so in this case 32gb would be optimal.