I think there is a real danger of taking the whole thing a tad too seriously.
Personally, I view all of it as a bit of a fun challenge, thinking "omg this will affect the next section of my life" hasn't really entered into my head yet. I'm nearly certain I'm going to enjoy my interview, I can't really imagine going in as a bundle of nerves and building it up to be something massive looming over me. To be honest, going in and having a chat about your proposed subject with some of the finest people studying it sounds like good craic. Whether that approach will convey my passion for the subject or not, well, I'm not sure.
Either way, when I weigh it up there a heck of a lot of great things about going and studying at Cambridge, but getting into my 2nd, 3rd or even 4th choice would mean doing a subject I really like the look of at a great university. In the end, it's about having a sense of perspective I guess.