Blu-ray is.... incredible. I have a 1080p Computer monitor, and I do have a blu-ray player in my computer. And I bought Harry Potter and the Deathly Hallows Part 1 and it has the DVD and Blu-ray version, and I put the Blu-ray in my player, and the DVD in a standard DVD player, and played them side-by-side just because I was curious, and it's amazing how bad the quality of a DVD is compared to a Blu-Ray movie. Again, this is on a computer, side-by-side comparison (no distortion due to the screen, they both were properly fitted to ensure fairness) with the Blu-Ray in a Blu-ray drive and the DVD in a normal average DVD player, and the DVD quality was just awful. It was, compared to the blu-ray, like watching it through a layer of mud.
So yes, Blu-rays will look amazing if they're blu-rays and if your Blu-ray player is decent and your TV is 1080P. As far as normal DVD's a lot of times the Blu-ray player will try to upscale the DVD to the proper format, so it won't look better.... but it probably won't look much worse either. Again, if you get a blu-ray player, just start buying Blu-rays, not DVDs.
In time DVD's will be phased out to a point, and all movies will be available on Blu-ray instead of DVD, much like how DVD's phased out VHS.