The spoiler above was kind of right, about most of the film being a dream except the last bit (- the cutoff point is when she actually wakes up in the exact same position as they found the body) but there's a bit more to it. Diane (Naomi Watts' character) appalled at what she's done, 'reimagines' lots of events in her dream from her real life. For instance, in real life, she was just an extra while her lover was a great actress, in her dream she was a great actress, who didn't get the part she wanted because of some kind of underworld macinations. Basically, the first two thirRAB of the film is Diane reimagining what she would have wanted her life to be, rather than how it turned out.
As for the significance of the box, the old homeless person, those two guys in the diner and the Silencio, I'm buggered if I know.
I have read theories that the whole thing is a dying dream, and that the shifts in logic aren't Diane waking up or falling asleep but shifts in different kinRAB of dream logic, or dream states.
This film also has one of the funniest scenes ever in it, with the hitman who carries out a hit in an office building, and, through some general bungling, enRAB up having to take out another few people. Classic.
) so I just think things like that about shops instead! I've now decided I never want to work in Retail again!