Well, I quit using VirtualBox for my virtualization needs. I am now using kernel based virtualization, or kvm. Kvm offers many advantages, in my opinion, the key being the simplicity and ease of use. I did not like the VBox abstraction.
- kvm’s core code is *in* the kernel, userspace tools are in a package. Kvm is part of Linux and uses the regular Linux scheduler and memory management. This means that kvm is much smaller and simpler to use. 
- kvm is Free Software released under the GPL.
- kvm uses processor extensions (HVM) for virtualization.
- kvm supports 64bit guests out of the box.
Also, kvm is a breeze to setup. In a nutshell, create the img.
kvm-img create gentoo-amd64.img 10G then boot the gentoo live cd.
kvm -hda gentoo-amd64.img -cdrom /path/to/iso -boot d. Then follow the gentoo install handbook. Simple.
Lastly, kvm images are easily transportable and will run on other hosts (within reason, can’t run a 64bit guest on a 32bit host, etc). As a matter of fact, I present to a amd64 guest if you would like to use it*, [here]. Unpack it, and it will decompress to 15-16G (sparse files). Then you can start it by simply running
kvm /path/to/img. In that guest you will find a pretty recent stage 3 install and emerge -e world done. There are a few other essential apps on there (vim & dhcpcd). From there, you can emerge xorg-x11, a DE, do whatever you want, really. I am using that guest to test out xfce development efforts and to be sure that we can bring you guys xfce-4.6 asap, once it is ready Cheers, and have fun playing with kvm!
*I do not wish to support this in ANY way, shape, or form. Use it how you wish, but do not expect help with it. Sorry.