I would say a personal thank you letter is above and beyond, and a gift is borderline inappropriate. You're entering into a highly professional atmosphere, and while I understand you want to express your gratitude to those that helped you get to where you are, they are working professionals who are doing their due diligence to perpetuate the standard of their profession. Similarly, you will have an obligation to recommend qualified applicants in the future. I've never rec'd anybody for graduate education, but I was point-blank told to never offer a gift (beyond buying a drink) to a recommending superior officer when I was still active duty. In all likelihood, they will appreciate the time you took to write a thank you note.
Just my experience, your mileage may vary.