In my test, using Chrome OS in VirtualBox was painfully slow because you can't enable graphical acceleration so it's best to use the USB images. But if you must run it in VirtualBox (if for instance you can't boot of an USB), it would be ideal to assign it around 2 GB of RAM.
To use the Chrome OS .vdi in VirtualBox, download it from HERE, extract it, then in VirtualBox click on "New", enter a name and under "Operating system" select "Linux" and for "Version" enter "Ubuntu":
On the "Virtual Hard Disk" step, select "Use existing hard disk" and browse for the ChromeOS .vdi file:
You're now ready to run Chrome OS in VirtualBox.