And i cant design my GUI for 800*480(w) because original design on my PC will be for 480(h)*800(w).I have to rotate my head to 90* for designing the GUI and all the text form. And its deficult. i hope you got what i mean to say.
You can size your widgets based on screen resolution. Check Digiflip example for mobile...it works in both landscape and potrait mode on a mobile.

Also may be you can cleverly use layouts and design on 480*800. The even if you change direction, things might work. Install the Nokia Qt SDK and check how they do it in simulator.