02-18-2012, 02:11 PM
The general purpose of RAM is to hold all the software that is currently being used.There is also a SWAP portion designated in the hard disk area that is utilized to handle any overflow. The physical act of pushing data to and from the swap area on hard disk is extremely slow compared to data operations performed only in RAM.
If you open Activity monitor and look under System Memory, you can get a feel for what programs or processes are running and can see what memory is being used.
As memory is full and disk swapping happens, programs slow down. This can cause programs to open slowly, long pauses, or the "spinning beachball". By increasing RAM, you minimize this risk.
For example, I have open on my machine running Lion (memory hog), iMail, iCal, iTunes playing music, iMessenger, LibreOffice with 2 open spreadsheets, Activity Monitor, Chrome (11 tabs open), and some very specific weather software. I still have 2.9GBytes of Free Ram available and ZERO Page Outs.
Prior to upgrading my Mac Mini, with stock 2 GBytes ram, I could have one program open at a time and even then, I was logging Page Outs with Activity Monitor. My performance was sluggish and barely usable. Now, it is operating exactly as I expected from a new Mac, crisp and responsive.
So, with all being said, I hope it gives you enough information to an informed choice as to your RAM needs. I would have to say though, if you are running Parallels, you need More RAM.