Boxes in the Attic

One of my favorite pastimes is to study interesting subjects online. Some topics have been found merely by hitting the random button on Wikipedia. For instance, the other day I studied blood, and found that blood types are not the only differences between people's blood. Someone could be unable to accept a donor's blood of the same type because of an Rh factor! Fascinating! So O- really isn't the universal donor.
Anyway, the point that I'm getting to is that in my quenchless search for knowledge, I am starting to find that I am...running out of room. For instance, the other week I was looking up malapropisms and related devices, and I can't remember what they are anymore. This forgetfulness has never happened to me (I of the elephantine memory). So I wonder, am I running out of room? Have all of the memories and facts of my 25 years started to crowd the hard drive in my brain? Or are my facilities simply aging and am I unable to retain things like I once could?
My favorite literary figure, Sherlock Holmes, would side with my first theory. In the first Sherlock Holmes novel A Study in Scarlet, Watson tells Holmes some commonly held scientific knowledge, along the lines of the Earth revolving around the Sun, to which Holmes responds that he doesn't care! His brain is only filled with things that are useful to him in solving crimes, and everything else is inconsequential. Holmes states that a brain is like an attic, it can only hold so many boxes before it becomes full and you can't find anything anymore, therefore he is a better detective by staying wholly ignorant of everything that is not useful to him.
Is this how our memory works, like Holmes' attic? Or can our minds be infinitely elastic? Most would agree it's not that simple. Memory, in its makings and retrieval, is so interesting. As interesting, I think, as forgetting. Isn't it a mystery how and why we forget things? Or are things never really forgotten, just repressed?