When running Windows XP in VirtualBox within a window, I noticed I could only set the resolution up to 800x600 (Display Properties | Settings), which had me disappointed. But, when I turned to a co-worker's screen, I saw a maximized VirtualBox window entirely filled with the Windows XP host and then my jaw dropped!
I set VirtualBox to full-screen on my system and I had the Windows XP guest doing a 1280x800 (the max. for my notebook's LCD panel).
I'm so loving VirtualBox now.