To avoid the drawbacks above I would suggest a hybrid option (that's what I am running):
machine is dual boot Ubuntu / Windows and I have setup virtual box in Ubuntu to be able to run the Windows disk/partition as a VM (need to create multiple hardware profile in windows - native & VM). There are multiple godo sites out there explaining how to do that.
This allows you to be running ubuntu + windows at the same time, and if for some reason , you did not get enough horse power (playing game or something like need special windows for special hardware) you can easily switch to native hardware, without having to maintain 2 images of windows.
Drawbacks: every time I switch back and forth I get the anoying windows registration, but I believe it can be disabled, I just haven't tried it
Other drawback is when installing windows on native hardware first is it will set itself up to use IO APIC - meaning you have to use that option in your VM which is a performance hit for VirtualBox. Unfortunately virtualbox docs clearly indicate we can't disable IO APIC in windows once it is installed. I am not sure if you can work around it by installing the OS in VM first (without IO APIC) then boot it from disk (haven't tried it).