Pretty much any TV that can handle a 780p signal and has either component input or hdmi input is acceptable. It doesn't even have to be widescreen as long as it fits those requirements.
When I first got my ATV, I didn't have a Widescreen TV or one with the right requirements for that matter. A couple of weeks later, I realized my 15" LG TV I had in my kitchen had component inputs and was HD ready (meaning it accepted 720p signal). I think that TV only cost me around $300, so I would think you could go to Costco or any Big Box outlet and pick up a cheap TV these days.
Mike...