Well, if you're not 100% certain that this specific girl will appreciate a letter, I'd say don't do it. There are a few exceptions, but generally most girls today to my experience do not like romantic guys, they take such sentiments as a sign of weakness, weirdness and desperation. It's a cruel and cynical world and you need to adapt to that, get her MSN or facebook or whatever and chat with her for a bit, make her get used to talking to you, then tell her you like her by being clear, brief and honest while skipping anything overly sentimental. Just my opinion.