RAM upgradation and Optimized use in Virtual Box.

I am using a Dell Laptop and have recently upgraded my RAM from 2 to 4GB, is there any way possible that I can utilize the total capacity of these RAM's? Should I use virtual box, for different OS or with this high capacity, I don,t virtual box? One more thing, is there some way possible, through which I can actually use more than one OS without using virtual box?
