Abhi as far as I know, virtualbox doesn't extend into products like cloudmin for example, so you'd need VMWare or XEN - neither of which are free or cheap. In terms of RAM, as much as you can get into the server, which of course needs to be a physical machine capable or visualisation. The amount of RAM you need really depends on how you intend to sell it.
Typically, if you have more then one physical host, 256Gb is a starting point, arguably more.
