For those who are avid movie collectors upgrading to Blu-ray can be a huge cost. The Blu-ray discs are great but are quite a bit more expensive than DVDs.
I have found that you can usually get good deals on Blu-ray discs online and that occasionally they will put up classic collections at a deep discount. I think if you have a list of say, your top 40 favorite movies of all time, these would be the ones to get on Blu-ray. Because of my huge collection of DVDs I only upgraded the movies I knew would look better in high definition. There are some, like romantic comedies, that I don't really feel need that extra graphic boost.