I have to admit I'm a closet gemstone jewelry afficianado. The number one thing, especially if you are willing to spend a considerable amount of money, is to involve your girlfriend in the purchase. Not only may you not get the style right, you may be spending money on one area, where she would rather spend it on another (for example you want to buy the largest stone you can, while she cares about the cut and/or clarity or color). Believe me it is far preferable to work together in picking something out that she will enjoy wearing (hopefully forever) than to make it a "surprise".
Second there is a website called pricescope that has information about diamonds so you can learn about them. You want to look for a stone with an excellent cut quality (that's really what makes diamonds so purty).
Third you will save money on your purchase by buying over the internet at a reputable site, with a stone with a gia or ags report, if you are comfortable with that. You can also find some beautiful rings in antique stores.
You can save even more by doing your homework and buying something off a resale site such as diamondbistro, or louptroop.
If you are getting a custom setting or purchasing via resale it is especially important to involve your gf as you cannot usually return in those cases. But, you will hopefully end up with a ring that you are both proud of, at a reasonable price.
If you do end up buying a ring in the thousands, either add to your homeowners insurance or get a separate jewelry articles insurance. Definitely worth it for the peace of mind, because the point of jewelry is to wear and enjoy.