Take it from your resident, happily-married for almost 12 years, 39-year old onii-chan: don't. Love is more than grand gestures based on sales propaganda from Jared and Zales. There are so many other things to use that cash on that is so much more important than a ring your future wife will be afraid of wearing anyway. Down payment on a house, nest egg, eroges, car, etc.
Simple gold bands should work well for both of you.