I had the similar issue in the Ubuntu 8.10 under VMware. I could only use it in the 800x600 resolution. But there was a tool called VMware Tools with the VMware. The program always recommended to install the tool within every guest OS's installed to get best support for keyboard, mouse and visual effects. After installing it in the Ubuntu and other OS's I have installed using VMware everything is fine and I'm presently enjoying the multy OS's experience as I have installed straight into the the hard disk.
I have never used VirtualBox. Just see whether you have something to install like VMware Tools in the VirtualBox. Or try to change the resolution in the Ubuntu in the Preferences section.
