First off stay away from Platinum, its way more expensive and will cost you more in the end. If she, like my wife, prefers silver get white gold. It will need to be replated at some point but that should be covered if you get the coverage plan. Some advice 1.) Find out what cut she prefers. Certain cuts will look better on her or she will prefer it. 2.) Dont get caught up in the size of the main diamond. I got my wife a 3/4 K main diamond with Baggots(sp) along the sides. It cost less then the main sizes but sparkles more then bigger single diamonds. 3.) Try to get a diamond just below a benchmark, it will be cheaper. For example get a .68 vs .75, you will never notice the difference but it will cost hundreds less. 4.) Get it appraised and insured right away. Also get it appraised every other year too as it will increase in value due to inflation. Then that way if she loses it like my wife did you will get more back then you would. Dont be shocked when the appraised is alot less then you paid for it. Stores mark up diamonds 2-300%. 5.) If you can get a set, Ring and band then do it. If she wants a band she will probably want it to be soddered to the ring. If you get the set you will be ensured that it will be able to be connected. 6.) **Cynical Post** Never give it to her on a holiday or birthday. If for some reason she breaks off the engagement you would have no recourse in getting it back if you gave it on a "gift giving" day. The courts reccogonize an engagement ring as a contract between the two of you for marriage. If she breaks up with you she technically is breaking said contract and you have the right to get it back. Hopefully that doesnt happen to you and thankfully didnt happen to me.