If you really want to impress her and you know her tastes ( this comes from looking at rings a little bit with her - you can look without letting on that you're actually going to buy it right away, if you want to surprise her), pick out a ring style and diamond style, cut, clarity, etc. by yourself. I know if may be cheaper online, but it's very helpful to have someone at a local store you can deal with and have the ring adjusted, clean, and maintained. I found a place where he helped me find exactly the design I was looking for and he explained everything about diamonds (cut, clarity, color, etc) and let me examine all of the diamonds under the scope to see which one looked better. This ring had one center stone and two side stones, and since he didn't like the way the side stones looked with the center diamond, he upgraded them for free because he wanted the ring to look its best.
I think you'll find that the smaller, local jewelers will be more willing to help you find what you need a reasonable price and they also take a lot of pride in their work (more than the large jewelry chains, at least). Now, if all your g/f cares about is the size of the diamond on the ring, then I guess you can forgot all of what I just said and buy the cheapest diamond that's a reasonable quality online. My fiancee is more into elegance than size, so take that for what it's worth, too. Hope this helps.
--Swish