On a 42" plasma from about a dozen feet - almost nothing to the human eye. Of course, processing takes a part of this... big time.
But, mostly, in a 42" plasma, unless you are close, you may not see any difference at all. Take a look at HD plasmas (1366x768) vs. ED plasmas (853x480) at your local BB/CC stores. Step really close, then step back to where you would actually be using it from.
Then decide if it is worth an extra $1,000+ for an HD plasma.