I became something of a jewelry shopping expert when shopping for diamond earrings for my wife this past Christmas. I think I can offer you some good advice on how to get a great rock for a great price, Wacky.
First, buying jewelry from brick & mortar retailers, and paying for all that overhead, is for chumps. Popular sites like bluenile.com have generally good prices, but the quality can be hit or miss because they don't actually have the diamonds in their possession. Bluenile is actually a network of sellers all over the world. Also, the packaging is crap - usually just a cardboard box.
Go to JamesAllen.com. They are a small, phenomenal online diamond retailer. They get excellent reviews, and the diamonds I got were GIA-certified and stunning, at nearly half the cost the brick & mortar retailers wanted for the same quality diamonds. I cannot recommend them highly enough. Box is beautiful (actually a smaller box shipped inside a bigger padded box, also beautiful). Oh, and they have a 60-day money back guarantee if you receive the ring and you're not happy with it for any reason. They also offer credit cards with 0% financing for 6 months. I guarantee you will be happy you went with them.
As for what to buy, here's what's important and what's not important:
1. Carat
2. Cut
It's debatable which is more important. You can get a small diamond that's very sparkly due to superior cut, or a big diamond that looks like paste because the cut is crap. You really need to aim for both. I strongly recommend you aim for at least 1 carat, and at least an "Ideal" or "Excellent" cut. In fact, you really have to get "Ideal" or "Excellent" cut, and then go as big as you can afford. As for what you an "afford" - don't be stupid but also realize that diamonds are really important to most girls and you want her to be proud of the ring forever.
3. Color
4. Clarity
These next two are far less important than cut and carat, and it's where you make some sacrifices to get a larger stone. For color, anything from E-I will be fine. In reality, you'll never see the color difference in an I unless you put it right next to something at least three notches higher.
As for clarity, VS1 or 2 is great if you can afford it, but it really isn't necessary. An SI1 will be just fine. Avoid SI2 or lower. No "I"s.
Ok, so that's how you get the best possible rock at the best possible price.
The tougher part is actually the setting - you really need her input on this. If you go shopping together, don't even talk diamonds - just figure out what setting she likes. You're already going to get an awesome diamond if you follow my advice. Major collar pop.
