The back-buffer is what you are drawing too. You should set your backbuffer size to a good value, i think the default is 800x600 which is not so good. Maybe try 1280x720, a standard HDTV resolution. If you play the game on a non-HD tv, the XBox will scale it down for you :)
In your graphics device manager, you should set the properties 'PreferredBackbufferWidth' and 'PreferredBackbufferHeight'. This should be done in the games constructor.
Mercury Particle Engine - Add impressive visual effects to your game!
www.codeplex.com/mpe