It all depends on your setup. Do you have a 1080p projector with 100" display? If so, you will notice a huge difference. Do you have a 720p 32" TV that sits 10 feet away? If so, you will not notice a difference at all.
I have a 50" 1080P TV that I sit about 7 feet away from. I notice a difference. I personally don't don't notice enough of one to update my DVD collection however. I will keep my DVD's, and if I buy new movies, buy them on Blu-ray.
The beauty of BD (and HD-DVD if it had won), is you don't need to upgrade your library. When S-VHS came out, I did not re-buy all my VHS movies, I just bought the new ones in S-VHS.







