I'm late to the party, but I have a word or two to add.
Of course, what you give her is an expression of your love for her, and she'll see it and know that, every day. So don't cheap out. On the other hand, no need to overdo it - as others have said, you can get better enjoyment out of your money in other ways, like trips.
Think of it in terms of purchasing speakers, where you want to find the sweet spot in terms of value. You could spend $200 on a pair of towers at Best Buy and save money, or you could spend tens of thousands on high-end speakers and get ultimate performance, but Axioms hit the sweet spot where you get almost all that performance for a fraction of the cost. You can get the similar results out of most purchases, including cars, washing machines (how's it going, mdrew?), and diamonds.
Make sure you don't get suckered into paying the retail amount on one of those diamond certifications or grading reports. Those can be helpful -
see here - but the suggested retail value (SRV) isn't what you should pay. Most places I've seen, for the range of diamonds I looked at, offer prices about half of that SRV.
Good luck, and congratulations!