if you want the dialog louder without making the rest of the movie too loud i think the best option is using dynamic compression (what cam suggested). your receiver might call it "midnight mode" or compression or something. it will make the quiet parts louder and the louder parts quieter. that way you dont have to turn it up as loud to hear the dialog. of course it wont be as dynamic, so the loud parts won't knock your socks off like they otherwise might
if your receiver doesnt have the option, your dvd player might have it in the setup menu.
