[ubuntu] Minimum Ram for UbuntuServer in VirtualBox?

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http://ubuntuforums.org – I'm trying a new setup for learning Linux OSes. I put the Debian Squeeze Graphical install on as the host OS on an old Desktop PC which has 1GB of Ram. I have installed Virtual Box and so far added Ubuntu Server to it and everything is working and updating. (I did change a Network setting from NAT to Bridge) The question of Memory is that I would like to install more virtual machines and run them at the same time if I can. I would like to have Virtual Box run Ubuntu Server, Debian(command line install), and CentOS. I'm learning these for a college project. Ubuntu Server wil (Hardware)