By the way, you should start downloading the image of Chrome OS from here, while I'll tell you what else you need to know. The image archivated, so if you are under Windows I would recommend you to download and install WinRar, and when the image download is completed, you can unarchive (or dearchive, or how it should be told) it. Leave the image as it is. In the VirtualBox, Google Chrome OS should be set as a Linux operating system, then as version Ubuntu. Everything you need to do is simple, and for additional informations you can either visit this link:
http://www.techcrunch.com/2009/11/19/guide-install-google-chrome-os/, or you can leave a comment and i'll help you. You will have to login into OS with your Gmail username and password.
I've tested it on a Macbook with 2 GB of RAM and Intel Core2 Duo processor at 2.00 GHz, and it worked fine.
Attention!! Do not allocate to VirtualBox more than your current free RAM memory, otherwise the computer can become unstable or it can stop responding !! (it would be good if you save your work before starting the virtual machine)
Thanks to WebUpd8 for everything.