You always maximize memory. Always.
System Monitor cannot be trusted. If you have 2GB, it will use below 2GB. If you have 4GB, it will use below 4GB. But trust me, if you go 8GB, it will use more then 5GB
4GB is "enough". But "enough" is WRONG. You have to go beyond enough. I have 8GB (max 2010-mini) and I usually saty around 5GB/6GB with eveyrhitng I want going on. Vuze downloads, Firefox broswing, iTunes playing, iMovie working in background.
I got amazed that one day I did hit 8GB, so memory should always be maximized, so you can rest at night and never worry about it again.