At risk of using the wikipedia at http://en.wikipedia.org/wiki/Eye the dynamic range of human eyes is about 1 million to 1 so that's as realistic as you can get. Less and you risk some loss in realism. So let's put a upper limit of about 2 million to 1 since we can't know what lighting exists in the room you will put the set in.
I find myself asking this question when it comes to the newer LCDs and plasmas that are out there, that claim dynamic contrast ratios from 50,000:1 to 1,000,000:1 all over the place, and I find myself asking this. How much is TOO much when it comes to making the contrast look too unnatural? I saw a few televisions in the store that claim to use a bigger number, but don't have the processing to back it up. To me, unless there's that equal balance between image processing and contrast, then the DCR means nothing. Somehow, I feel that there has to be a line drawn when these numbers will just make a picture look too fake, and when the balance is set in place that it will look almost like the source. What are your thoughts about the two factors? There's also rumours that the glossy screen on the A650 just adds too much to even make for a natural contrast, if that's even true.