I once had an employee leave for a great management job elsewhere, we had just put him through a long training program and were happy with his work, and were sorry to see him go, and we had spent a lot of money on him lol. But I liked it when he wrote a note to the general manager complimenting me and the other dept managers, saying it was a great place to work, my group had great morale, blah blah, but he couldn't turn down such a great opportunity. Much better than saying this job sucks, I'm leaving for something better..although that's exactly what he did, and I understand!

I think you actually have a better excuse for leaving than most! Most of us have to finesse a nice way to say, "this job sucks, I'm leaving for something better."

In my not-extensive experience with job changes, I have found most bosses to be very reasonable. Most people I have a good working relationship with care about my own career development, not just what I can do for them. The only times I got any flack were from bosses I already thought were jerks.

Have the above conversation in person. I've always heard that your resignation should only be state that you are resigning, effective X date.
Totally agree not to burn any bridges, and certainly resist the urge to leave a paper trail with your resignation letter, as tempting as it is!