VirtualBox is a good solution to have occasional access to Windows, or to be able to quickly run a Windows application without having to reboot. It is also a good solution whenever you need to rapidly share your data from a Windows, and from an Ubuntu application.

But there are some drawbacks from a virtual machine. As it is true that the performance of mere data processing applications or editors will be more than sufficient given the specs of your machine this is not true for graphic intense applications.

I have no experience how much the flash development tools make use of your graphics card but I suspect they do a lot. In this case you may not be happy with the somewhat sluggish performance in a virtual machine. It may be good enough for testing, but probably not for application development, where it is also critical to not introduce bugs from the virtualization environment.

Therefore I would go for a dual-boot setup where you boot Windows for development, and Ubuntu for safety.

But I might be wrong, it may be that VirtualBox performs good enough for you, and on your individual machine. You need to test this, really. Intall Ubuntu as dual-boot with your presumably already preinstalled Windows. It is not much of an effort to also install Virtual Box to run Windows in Ubuntu ¹ . In case you are happy with the performance, just leave Windows sitting there for an occasional boot, or remove it later.

¹ _{Note that for running an additional Windows in Virtual Box you will need a separate own licence.}