While a standard Ubuntu desktop install uses around 400-700MB you could try a Ubuntu Server install. The overheads are drastically reduced as there is no GUI. I do not have any expereince with other linux types so cannot say in regards to their effectiveness.
Do you have to use VirtualBox? Consider using a bare-metal virtulisation package such as ESXi (VirtualBox, on the other hand, needs a OS installed on it already).