I would greatly discourage buying diamonds or other jewlery off the web. You can not appreciate the important characteristics of a stone unless you see it in person. What you need to do is to get knowledgable about the four C's, and realize that in order of importance they go: Cut, Color, and then either Carat / Clarity.
Buying a diamond off the web is like buying an used car without ever test driving one. Greatly discourage it. And remember, mall jewlery stores mark up their diamonds around 1000% and they will negotiate. You just have to be steadfast and don't buy into their bull. Also, don't let them convince you that the color is not that important. They will try and sell you a larger diamond with lower color quality and their lighting will make it look great. Trust me. Your girl will be embarassed later when she compares her stone with someone elses.
Not sure where you live, but I would recommend that you find a Merchandise Mart that is in your part of the country. These are where the jewlery stores go to buy their diamonds. The general public can not get in without knowing somebody - usually. However, call one of the vendors and say that you have purchased from them before and you have X number of dollars to spend today on a ring (dont go unless you are ready to buy) and they will arrange for you to come in and buy. This is what I did and you will be amazed at the value. They will sell you whatever you want, let you pick out the stone before it is mounted, at a jewlery store price (only about 200% mark up). Although I am no longer married, I went to the one in Atlanta, GA and spent $3000 and walked out with an engagement ring, with an appraised value of $8000.00 It has a .85 carat round brilliant stone, and absolutely colorless. With the bagettes, the carat total is 1.5. I will let you know what I sell the ring set for later this month. Going to go to a jewler who purchases estate jewlery.