I hoped this was a joke topic, but I fear its not.
The improvements on these blu ray releases for older movie is unreal. I think it actually makes way more sense to release on blu ray much older movies (pre 1960) than the new stuff.
Comparisons between DVD and bluray for:
The General (1927)
Gone with the wind (1939)
Hamlet (1949)
North by Northwest (1959) Since you asked
Seventh Seal (1957)
There are actually tons of sites dedicated to reviewing blurays, in terms of audio video quality and extras.
Ya, I posted before I really looked. I spent the day looking at what's worth buying.