Virtualbox, Guest Additions and Screen Resolution
This is sort of a well known trick but I'll post it anyway.
The problem is you install Virtualbox (My Host: Fedora 18 x86_64, My Guest: Windows 7) and the guest screen resolution is somewhat limited. Maybe it's 1024x768 or something like that and you can't get the full screen capabilities of your host device.
The solution - install Guest Additions on your guest OS (Menu | Devices | Install Guest Additions ..) and then auto resize guest display (Menu | View | Auto Resize Guest Display).
Now everytime you resize the window in the host the video driver in the guest will "see" a different monitor it can display on and adjust its options accordingly.
You can see this in action by right-clicking on the guest OS and selecting "Screen Resolution" and then watching the choices change as you resize the Vbox window on the host.