Agree with the above - i think a lot of xbox games are coded in 4:3 display format, so even if you have your xbox at widescreen setting it will just stretch it out a bit, not add pixels. However, it stretched too much the aspect ration would look funny and might even be blurry. You can see this at times in burnout revenge.
However, some (like halo2) are coded for widescreen and therefore looks ok even if it is pulled out a bit.
This is one of the good things about the xbox 360 being native widescreen as well as high def - because its not just the verticle pixels that are important (about 200 extra)(this gives the 720p or 1080i figure) , the horizontal ones need to be increased much more due to different shape (about 600 extra) or the picture won't fill the horizontal aspect.