I haven't been playing WoW much - bought Borderlands on a Steam special last week - been playing that. Also, I check the forums everyday, but don't have anything constructive to add to them
Have you tried to set the wallpapers to scale? The aspect ratio of the resolution should in that case be the same. I haven't got much experience with laptops and this issue, but often the screen resolution of the external screen is much higher. If the one is widescreen and the other one is not, then you might have some problems. The different resolutions are different aspect ratios. If you divide one number by the other you will see. 1280 / 800 = 1.6 while 1600 / 900 = 1.77777777
Apologies for only seeing your request so late. I am not every day in the forums either and with two jobs that tends to be difficult.
The graphics card has got two plugs for output. The other option is to have two graphics cards. My last couple of cards all have had two. At my work, all twenty PCs have two screens. When it comes to games, I mute the game and set the player on my second screen to fullscreen.
With your laptop, you have to extend your display to the second screen. Games normally always come up on primary display and do not always like to go fullscreen on the secondary. Windows has got an identify option. You either access this through your graphics card control application thingy that comes with your drivers, or right click on your desktop and click on properties, or Fn and some function key. but then you should normally have to have the screen plugged in. Windows XP sometimes insists on only detecting the second screen when you have it plugged in at startup.