TV Picture diffenences between HD and non HD

I am considering the Sony 42" WEGA LCD Rear Projection HDTV (KDF-42WE655) but I am confused about the display quality for non high definition stations. Will this TV display the non high definition stations okay or do I have to stretch the screen for full display and, therefore, the picture image will be distorted? What does it look like if I do not stretch the screen? Please explain the picture display differences between the high definition stations and the non high definition stations? Thanks.....