1.) You run both screens in 800x600, rendering over 20% of your netbook's internal display useless.
2.) You run the external screen at 1024x768 and the internal screen at 1024x600 and have the bottom 168 pixels show up on the external display, but not on the netbook.
Something many people don't know about is that the display tool xrandr has a really nifty "scale" option you can utilize to make your netbook's screen behave as though it has a higher resolution than it really does. For instance if you have the second case I mention above you simply need to run the command:
xrandr --output LVDS1 --mode 1024x600 --scale 1.00x1.28 --panning 1024x768
in terminal (note your internal display name may vary, most laptops default to LVDS1 though). After running this you will see your netbook display adjust itself so the height of 768 is smushed into it's normal 600 units. This appear a small bit stretched in this mode, but so long as you don't get too crazy with the scale factor it is hardly noticeable.