I recently installed Ubuntu 8.04 on my virtual machine (I am using
VirtualBox as my virtual machine) and I can't change the screen
resolution. When I try to change it, the only two options I get are
800x600 and 640x480. I want o increase the resolution. Is this a
problem with Ubuntu or with my virtual machine? How do I get Ubuntu to
use the host OS video card?

Hi there,

I had the same "problem" too.
The problem was that the guest machine was using incorrect refresh
rates, probably because it was using default/failsafe/conservative
settings, or virtualbox was emulating a substandard monitor, do'nt know.

Cure: simply edit /etc/X11/xorg.conf in the Ubuntu virtual machine, so
as to explicitely specify the correct refresh rates for your monitor.