When it comes to HDTVs, I believe the minimum for screen sizes to notice a difference is 37" or higher. However, for plasma, I don't think they make them smaller than 42 inches.
Its true are far as im concerned due to that most plasmas run in 1024x768, 720p which is crap if you ask me . Sure plasmas are known for better black levels and 600 hertz refresh rates but modern LCD'S that are able to run at 1920x1080, 1080p 240 hertz are kicking butt and now with LED'S oh yah.
It makes a pretty big difference. Just think about it..your average DVD is 5-6 gb of data...average blu ray is 30-35 gb...yea theres a bit of difference in picture quality.