Starting with Version 2.0, VirtualBox also supports 64-bit guest operating systems, under the following conditions:
You need a 64-bit processor with hardware virtualization support (see Sectioná1.2, “Software vs. hardware virtualization (VT-x and AMD-V)”) and a 64-bit host operating system.
You must run a 64-bit version of VirtualBox on that OS (Windows Vista, Linux or OpenSolaris). This can then run both 32-bit and 64-bit VMs; a 32-bit VirtualBox can only run 32-bit VMs, regardless of the hardware.
You must enable hardware virtualization; software virtualization is not supported for 64-bit VMs.
On most systems, the hardware virtualization features first need to be enabled in the BIOS before VirtualBox can use them.
There is no specific setting to enable 64-bit support for a guest. If the above conditions are met (in particular, if hardware virtualization is enabled), 64-bit support is available, and you can simply install a 64-bit operating system in the guest.
You should enable the I/O APIC for virtual machines that you intend to use in 64-bit mode. This is especially true for 64-bit Windows VMs. See Sectioná3.7.1.2, “"Advanced" tab”. In addition, for 64-bit Windows guests, you should make sure that the VM uses the Intel networking device, since there no 64-bit driver support for the AMD PCnet card; see Sectioná6.1, “Virtual networking hardware”.