I have developed a xna game based on the resoltion 480x800, how can this game be targeted other resoltions like HD720P (720x1280) or WXGA (768x1280).
When running on Windows Phone 8 devices it will automatically be scaled by 150% or 160% (for 720P & WXGA) as appropriate.
As it's not possible to create XNA games that directly target WP8 you cannot create an XNA which uses the 16:9 aspect ratio and so you cannot make use of the full screen of a 720P device.