I agree with kurtz. Language is constantly evolving. It is a good thing, IMO, to challenge one's self by reading older books with period words (like Pride and Prejudice with it's words like 'felicity' and 'abhor') because not only do they help expand our vocabulary, but they also add color and help set the scene for the story. Certain stories wouldn't have as much magic if they were written (or translated) into modern English.
One of my favorite books of all time, Pilgrim's Progress by John Bunyan, was written in the 1700's and has been translated several times into modern English. Sure the story is the same in the end, but only the original version feels like the story the author intended it to be. (Sorry for the long rant. )
I say keep reading the classics even if you need to look up words because eventually (and you may not even notice at first) you'll start ignoring your dictionary because you'll have learned so much.