If the seventh book has three acts, the second would begin with the trio going to hide in the forest, and the third would be all the stuff at Hogwarts. So the first movie pretty much ends on a somewhat cliffhangery note while the second act is still going on.
Most of the HP dvd's look pretty good, but the sixth definitely got shafted on the transfer. While the Bluray version is said to be excellent, the dvd is inexplicably soft, way too dark, and desaturated. It was a bit dull-looking in the theatre anyway, but the standard definition image quality is dreadful for no apparent reason. A fair amount of Warner Brothers dvd's have been rubbishy in appearace lately, which probably speaks to the studio just not caring about the format anymore. This is very unfortunate for those of us who are not yet in the HD game.