In todays computers you cannot switch as you like between resolutions, even if your graphic adapter supports it. This is due to several limitations in your graphic display. Because your computer is a laptop, I assume that you are using the main display which is using a LCD/TFT, plasma or OLED. This display it was not designed to support all the resolutions that your graphic adapter might support. If the Desktop click right -> Properties -> Parametres doesn't show the 800x480, it is because the graphic display (or the graphic display driver) does not support this resolution.
In order to achive this resolution you can:
Option A. Connect to the VGA/HDMI output of your laptop a display that supports this resolution.
Option B. Write on regular A4 paper 800x480 and stick it on your screen .