Sounds like potential profit, sure.
I have one issue with that <i>particular</i> necklace. While my ever-so-charming wife lurkette is a HUGE LOTR fan, she's actually much more a Tolkeinist than a Peter-Jacksonist. So anything that elevates the character Arwen from her fairly minor role in the novels is strictly out of bounds.
If your girlfriend is more into the books than the movies, then I wouldn't recommend that necklace. If she's into the movies, fire away. Sounds like an inexpensive (don't say cheap!), pretty, sweet gift.
Now, regarding this promise ring... Don't take this wrong, but I've always thought that was a pretty dumb thing. "Are you engaged?" "No, we're promised..."
It sounds like you're 6 years old. Oh, and a simple silver band usually means you're <i>married</i>, not "promised".
I say you either get engaged, or you don't. You don't need some big fancy expensive ring to do it, either. I bought lurkette a $29.95 cubic zirconia that she picked out of a catalog, because we were college students who couldn't do any better at the time. It was pretty and meaningful <i>without</i> costing an arm and a leg. And when I was able, I replaced it with something better.
All that said, I'd recommend NOT combining her birthday with a marriage proposal or even a "promise" proposal. Both are special things, best to keep them separate so they stand on their own.