Another thing to note about memory. If you are using a hypervisor the memory overhead for the 'OS' is extremely small. IIRC, on the ESXi box I set up, ESXi I don't think ever used more than maybe 64MB of RAM. Other than that, I would say plan at least 256MB/VM if they're running headless *nix, 512MB/VM if they're running GUI'd *nix or Windows XP/2003 or earlier, and 1GB/VM if they're running Vista/2008 or later. So, if you want to run 5 VMs, I would throw on at least 6GB; might as well do at least 8GB with the prices of 4GB DIMMs where they are now.