For those using Netflix via their Xbox 360, they are members of Xbox Live (which costs $50 per year). Adding Netflix is just an added bonus, but I would not recommend someone join Live just for Netflix. If you're not gaming, the Roku would be a better bet. You could also purchase the wiring to have your TV double as a 2nd monitor to your computer. You would then just load the movie via the computer and watch it via the TV. I'd imagine any new HD TV would do the trick, but if you've still got an old TV that you intend to use, I don't know if that would be an option or not.
You don't see a lot of people talking about the costs involved to get the Xbox set up for Live because that's an expense most of us paid long ago. For us, there were no extra costs involved with setting Netflix up to view via that route. It was just a plus/plus kinda deal.