What Is The Ideal Configuration For A Computer To Run A Vm Smoothly

If I want my new computer to run Windows 7 or Ubuntu in a VM (VirtualBox) -flawlessly-, with great speed, full-on graphics (including Aero), and no major impact on the “real” OS (the host) – what sort of hardware specs should I go with?

What are the components that have the most impact on VM performance, and assuming money is no object, what should their optimum specs be?