I've gone over to a Mac primarily for Garageband and logic. I'm not that competent with computers but was very happy to get VirtualBox an Windows 7 up and running. I'm a bit stuck though in how to assign more graphics memory to it as it slows down when I open a few pages.
Firstly make sure you have installed the Virtualbox Guest Additions for the guest operating system. Then, to increase guest video memory select the guest in the Virtualbox Manager and then select the Display Setting in the menu list (click on word Display in the right hand panel). Then you can adjust the display memory. Have to say I normally leave display memory at the default size and it seems to work ok for me so this may not be the cause of your problem.
There is a very good manual here: http://www.virtualbox.org/manual/