Dont expect to much from your gift, expecting her to cry is a lot.
If she knows you like her, and she likes you then why is she still with her BF?
I wouldnt listen to much of what "bzborow1" said that might have worked with the girls he's been with, and all the girls he knows might be like that. But thats stereotyping women, and not all women are alike, because i dont personally agree with anything he said, not trying to be mean, just dont want you to think like that!
I would suggest being yourself, sounds like you know girls like being treated good, so for the "special day" take it up a notch, be extra sweet.
Basically just be there for her, dont pressure her or anything!
Just some suggestions!
Good luck