If you're using KDE, the easiest way would be to right-click a blank space on your desktop background. Select "Configure Desktop", click on the "Display" option from the left column, then change your resolution on the right side. You shouldn't have to edit xorg.conf directly.
For future reference though, to save the file you're working on in vi, while in command mode, type ":w" without the quotes. Think "w" for write.
Hope this helps!