Let's say I power on my fresh installed Windows 7 x64 machine. After Windows boots up, there are a bunch of services being started in the background that start allocating memory.

Then I enter my user/pass and Windows logs me in. Let's supose I don't do anythig else (I don't explicitely start any application) and I don't have any other app installed by me. So it's fresh install of my machine.

My question is: how much memory is needed for all the UI & other stuff? Is it a good indicator to look into task manager and check all the processes started under my user name and sum up all the memory consumed by those processes to get the total amount of memory I am consuming just to stay logged on? Basically this is my question: how much memory is needed just to stay logged on?

Now, if log off would all the memory be released back to the system so that the background services can benefit of?

Also, I assume that there might be a different discussion for each Windows flavors (?)