I have an Ubuntu 9.04 guest running on a VirtualBox VM hosted by MacOS X Tiger. I'm trying to increase the screen resolution as the guest by default unfortunately supports only 800x600 and 640x480.
I have done some research and also studied the answers to question 1660 on this site. I understand that the change of screen resolution requires the installation of the "Linux Guest Additions". As described on this page, I tried Devices>>Install Guest Additions from the menu of the VM.
The according CD image was properly mounted to the Ubuntu desktop but after running "sudo bash ./VBoxLinux" in /media/cdrom all I got was "Detected unsupported X86 environment".
According to the VirtualBox manual it should also be possible to resize the VM window and thereby change the screen resolution of the guest. The menu item Machine>>Auto Resize Guest Display is activated and grayed out. Changing the actual window size of the VM window does not show any effect on the screen resolution of the Ubuntu guest.
This kind of concludes my effort so far. I'd like to know if someone managed to increase the screen resolution of an Ubuntu guest running on a VirtualBox/MacOS X.