Looking to keep it under 5 gig, huh? If a server edition will work, ie no graphical interface, Ubuntu Server edition will come in under well under that.
I have a copy running dual web servers including SSL and PhP, MySQL, and Postfix mail, Bind, and SSH, and mdadm for software RAID. It currently weighs in at 1.47G. I haven't installed it inside VirtualBox, but I did consider it as it does play nice with VBox (there is even documentation on how to set it up).
By way of comparison, the laptop that I am using now with the full version comes in at 6.5G on the root directory alone.