in my opinion if you have a dvd player with an upconverter, the picture quality doesn't change too much, blu-ray is still an improvement. The sound quality is where blu-ray excels. If you don't have surround sound you wouldn't notice the sound improvements though.
DVD is standard def, BluRay is 1080. Same difference as going from SD to HD.
Try watching the standard def version of ABC, CBS, NBC, etc. tonight. Watch for a good 20 minutes to get used to the SD picture. Then mid show - switch to the HD version. This will show you the difference between DVD and BluRay.