Enable 3D acceleration in the display properties when you set up the guest. After installing the guest system just add the guest additions iso found in usr/share/virtualbox. The iso will show up in the guest os as a mounted CD-ROM. Double click the the mounted volume and run the script. Now you can run Compiz, Unity, etc and you are no longer stuck in in 800 x 600 resolution.
If you have an earlier version of Virtualbox installed make certain to remove it and all traces of it or you may run into problems getting 3D acceleration to work.