By checking the Ubuntu forums, I found that I am not the only one to have problems with the login screen resolution. It seems it is a well known bug, but nothing has been done to correct it. So, as with many things Linux, you have to edit a system configuration file.
Before we begin, a word about shop safety. Do not attempt these changes if you do not know what you are doing. Changing any system configuration file may render your PC non-functional. YMMV. Insert disclaimer here. There be dragons here. Instructions pertain to Ubuntu 8.1 and ATI 7000/VE video card on a Dell OptiPlex GX260.
First, backup the file you will need to edit by opening a terminal window (Applications/Accessories/Terminal). In that window, type the following:
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bak
Assuming this succeeded (which you will not know about unless you check after typing the command above to see that a copy of xorg.conf is now in xorg.conf.bak), you can, while still in the terminal window, type the following command which will allow you to start editing the xorg.conf file with administrator rights (that is, the system will allow you to save changes to the file):
gksudo gedit /etc/X11/xorg.conf
You will probably be asked to enter your password. Do so. Once the edit screen appears, scroll down to the section that looks likes the one below. Note: I have snipped off part of one line because it would cause problems in displaying on my WordPress blog. Your actual line will probably extend for some length.:
Section "Screen" Identifier "Default Screen" Monitor "Configured Monitor" Device "Configured Video Device" Defaultdepth 24 SubSection "Display" Depth 24 Modes "640x480@75" "800x600@56" "640x480@72" [SNIP: edited to fit the width of my blog] Virtual 2560 1024 EndSubSection EndSection
Once you’ve found this section, put your cursor between the word “Modes” and “640X480@75”, or whatever your first resolution mode is. At that spot, type in the resolution you want your login screen to default to using the format as shown (e.g., “800X600@56”, remembering to include the quote marks). Note: the resolution MUST match something both your monitor and video card can work with. Failure to do so could cause your monitor to fail. What I did was copy the resolution that I wanted from the long list and then pasted it at the insert point noted in my instructions. Once you’ve done this, save the file, exit the editor, and exit the terminal window. Then, making sure you don’t have any other application open with unsaved data, logout of Ubuntu (or reboot) and check to make sure your login screen is now running at the resolution you changed it to. If so, you are done. If not, and once you are logged back in, open a terminal window and copy your original configuration file back:
sudo cp /etc/X11/xorg.conf.bak /etc/X11/xorg.conf