flyseye Felix4067 is correct. 16:9 for HD channels. Dont know why non-HD channels even exist anymore. For the vast majority of the population like myself that cannot afford several hundred dollars for a new TV, or the $100+ increase in our monthly cable bill so we can watch HD channels only.
You know, you can get full HD programming, absolutely free, over the air, using the cheapest set of "rabbit ears", from any broadcaster who offers it.
Granted, you won't get any "cable only" programming, but If you're like me, and refuse to pay for cable anyway, it's the way to go. If you can't afford the new TV, you can always get a converter box to hook up to the old tube model. One of the options available is for "letterbox" viewing, which will give you the 16:9 aspect ratio.