The virtual resolution will be as large as the largest screen resolution listed (absent a Virtual line). So, if you delete 1280x1024, leaving 1024x768 800x600 640x480 you'll get a virtual screen size of 1024x768 no matter what order the entries are in. You can specify a larger virtual than the display resolutions. So if you had 1024x768 as your largest resolution and set Virtual to 1024x1536 you'll have a virtual resolution that's as wide but twice as "tall" as your display.
Viewport is a different, but related thing. It affects the initial position of the display when the virtual resolution > display resolution. Good for getting xdm|gdm login centered, I guess. -- ¶ One·should·only·use·the·ASCII·characterset·when·compos » ing·email·messages.
