VirtualBox, and possibly other VM software, allows the user to specify the number of CPUs exposed to the VM. In my particular case, I have a dual core CPU, with Windows 7 64-bit running natively, and Ubuntu 9.04 64-bit as the VM.
The help file suggests not setting this above the number of physical cores available. This makes sense. So, is it best allocating 1 or 2 CPU cores? Does this depend entirely on the native CPU load?