I haven't used VirtualBox in a while, but the last time I did it wasn't as good at emulating from a physical partition as VMware, which isn't that great either unfortunately. That's a shame because it's much better to emulate from partition: in theory it should be more stable, the disk I/O should be faster, and if you ever need more speed you can just reboot (changing hardware profiles, etc) and what was guest now is the host, and vice-versa.
Inter-OS virtualization tech still has a ways to go, and
CoLinux has been a huge disappointment so far. Won't it be nice if you could not just map a drive to the other OS but also run its executables better than wine /
faster than cygwin. I don't mean see the apps on the desktop (ex
Xming), I actually mean share the command space, like you could add Z:\usr\bin to your Windows path and run wget from PowerShell or gcc from Visual Studio!