please read http://wiki.linuxmce.org/index.php/Custom_Display_Resolutions
, a tutorial which explains how it works.
Your resolutions (480p and 1080i) are hidden in the /usr/pluto/share/Resolutions.conf file. Using this guide, change the visibility flag from the desired section (resolution) and run AVWizard again.
If your TV is a LCD or plasma one, you should use the useEDID, set it to true after running AVWizard and applying the ugly hack (Prevent LinuxMCE from changing the Xorg config). This will (in case of using a LCD or plasma TV) solve the overscan issue, if you get it.