I switch games a lot.
Typical session would look like this :
Play Victoria2. Got bored.
Alt-tab to Windows. Start Crysis2.
If I had more than 8GB RAM, I probably would have a lot more games open concurrently just because its so convenient to switch around instead of having to wait for them to load. The games themselves are not that memory hungry, however you add Firefox and Chrome with 30+ tabs open, Eclipse, Xchat, Torrent client, Songbird media player, Skype, Anti Virus, Firewall, Apache,Postgresql, etc. running at the same time and you suddenly looking at 7GB+ memory used.
Unfortunately some games suck CPU time even though they are running in background.