I'm installing Ubuntu in a Virtual Box; my host OS is Windows 7, and I have 6GB of RAM on my system. One tutorial on setting up Ubuntu on a virtual machine (this one, on step 14: <http://www.aztcs.org/meeting_notes/linuxsig/ubuntuintovirtualbox/ubuntuintovirtualbox.htm>) says to set the RAM at 45% of the total RAM I have available, which would be about 2.7GB in my case. Is this correct? 

Also, when I allocate RAM to the vm, is that RAM now reserved exclusively for the vm, and if yes, is it only reserved for the vm when the vm is running a guest OS or does it create a permanent division in the RAM, i.e., when I stop running and quit from the virtual machine, will my host OS (Windows 7) regain all 6GB of RAM as its available RAM?