Once you install XP into VirtualBox, there is an option to install guest additions. I believe the hotkey is HOST+H or HOST+D. Not sure off the top of my head. You must boot into safe mode to install the 3D drivers.
Also, the 3D driver, when installed correctly is light years faster than BootCamp due to allowing hardware access. I have played UT3 in VirtualBox on XP SP3 with no issues, and it is much more intense graphically than SH3.
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