On my system, with basic stuff open (vlc streaming public radio, file browser, terminal window, system monitor) the system uses a bit less than 1GB. With Chrome open and a couple of tabs, about another 300MB. With a Windows VM, add another 2GB. I always figure that it's best to have about 2x what you use in normal conditions, so in my case that would be 6-8GB based on a normal usage of 3-4GB RAM. The only time I have hit the swapper is when some buggy process runs away with a major memory leak, which is the reason for running the system monitor. With that I can see that when the memory usage starts to go out of range, and kill the offending process before the system freezes from excessive I/O on the swap space.