Curley's wife's dream of becoming a famous movie star is suggestive of at least two things: 1) she is bound to be disappointed with the reality of her life and 2) she is just as conventional as the rest of the characters in terms of their world-views.
The dream that Lennie and George share is a touching yet somewhat stereotypical dream, perhaps it draws its power from its very commonality. It bears the logic of the obvious. Of course they want to own a place of their own and work less. Everyone wants that.
These dreams serve to connect the audience with the characters and to elicit sympathy as well, as the audience can both relate to the dreams and see them for what they are - innocent visions of a better future which are unlikely to materialize.