OP Be warned it's not that easy to say ' do old films look great on Blu-Ray' because at the moment a lot of companies are putting out 'Rush Job' versions, that are pretty poor HD versions that look little better than the DVD release, but have been thrown out there by studios wanting to bag a quick sale on the new Blu-Ray format. These studios are then hoping to do a proper transfer a year or two down the line so they can call it the definitive version, and get people to pay for the movie again.
So often it's a minefield of classic movies that look excellent and use the full Blu-Ray Technology, and weak Rush Jobs that look poor and hastily done. So go to an independent review site before you buy.
Some Great classic Blu-Ray Transfers:
The Wizard of Oz
The Red Shoes
Casablanca
It's a Wonderful Life
Some bad 'Rush Job' ones are
Butch Cassidy & The Sundance Kid
Night Of The Living Dead
The Battle of Britain