I went with this same idea when I upgraded. I had a limited budget but wanted to get a few of the best and from my testing I found these worth picking up.
Shawshank Redemption - I picked this up for $8, it is probably my favorite movie so I had to have it but I was blown away by how great it looked over the DVD.
Watchmen - Another I got for $8 and worth it. Great movie and very noticeable difference in quality over DVD. With the FX in the movie it is a must have.
Kick Ass - Again, $8 and just a solid flick. Has several dark scenes that are much clearer on bluray and with a good HDTV. On DVD they seemed a little blurry but not on the bluray.
Inception - Obvious choice and another $8 grab for me!
Those were the fist blurays I got. All on sale when I got them. Some others that on my "Want List" are:
Star Wars (original trilogy), Matrix Trilogy, Avatar, Robocop, the whole Marvel series (Iron Man, Avengers, Etc), E.T., Indiana Jones Trilogy, and Saving Private Ryan. I could go on forver with the list but those are kinda the ones I want most.
Gratz on the upgrades btw!