I want to configure a system for cross-browser testing of rich internet applications in browsers. My requirement is to have one system with 20-30 virtual machines (eg, with VirtualBox). There will be a need for up to four concurrent virtual machines each running tasks like: automated scripts that drive browsers, humans driving browsers, image processing, a JVM, etc.
So to summarise that's 20-30 VMs with up to four concurrent VM sessions. What kind of specification machine would suffice?