I'm a girl...and unless this guy is my bf of at least 1 month...I would FREAK OUT!!!!!!!!!! (in a bad way).
Letter like this are quite intimate. Would you write a similar letter to a male friend who is also very close to you? If not, then you should not give it to a female friend.
I suggest you get her a more neutral gift (NotesMaeve's suggestion of iPod was great), with a warm card. But...when you give the gift to her, tell her the stuff you wrote in your letter. In letter form, it's too intense. But spoken, the words can sound much more causal and platonic.