Having trouble picking out the perfect ring? What if she picked out the exact ring she wanted? How's this for the best article of the century! Just let your future wife pick from the internet diamond stores! There are so many online that let you create your own ring. She can pick everything for details and you will know exactly which ring to get! I used bluenile.com even though they did deny my affiliate application!
http://www.bluenile.com/all_settings.asp?filter_id=1&track=design_state&setting_search=1

I believe your future fiancee should know your planning to propose but it should still be a secret. Using this method of picking out the perfect ring she gets exactly what she wants and you don't have to go to the store and try on rings for a day or two. Once you take the time to visit the jewelry store she is going to expect a ring real soon. Using the internet makes it more of a dream and not reality just yet!

Another option is to have a fund set aside for the ring and pick up a promise ring of some sort. Something she might wear after the real ring shows up. You could give her the pre engagement ring as an engagement ring and then break the news to her that this is just a filler ring and she has $X to spend on the real ring. This way she gets the ring she wants. Remember it will be on her finger for the rest of her life!