How does this fit in with the letter you are thinking of giving? I like the sentimentality aspect, but I think you are really sending mixed messages if you do both of these in a relatively close window. If I would do this for my wife, I would assume that she would really like it and it would make her feel that, to me, things are going well in our relationship. If I then turned around a short time later and gave her a version of your letter, I think she would feel like the ring was not genuine and that I was just messing with her emotions.
CB
Me; 42, W; 43 M; 16 yrs S12, D9
3/13 - "I want to move to XYZ City (four hours away) and it might be without you, not sure" 5/13 - "Not sure I meant that"