This is a perfect exemplar of something I really hate about this website. A poster asks for advice about how to buy a diamond, and instead he gets mostly replies saying "don't buy a diamond." I will try and actually be helpful.

My advice would be:

• Your girlfriend probably has much stronger views than you do about jewellery, and after all she will be the one wearing it. Propose with a "fake" ring, then go shopping for the "real" ring together. I got a very nice-looking ring off Amazon for £10 to propose with. This minimises the chance of making a bad decision, and is also a romantic thing to do together.
• If you do insist on buying the ring beforehand, make sure you can take it back. Many places will do returns within 30 days. Borrow a ring she finds comfortable to get the sizing.
• Do not get hung up about high degrees of quality. VS2 clarity and H colour is plenty. She will never tell the difference between having a VS2 and VVS1 diamond on her finger - these differences are only visible when put next to another diamond in the right light, which will never happen.
• But make sure the cut is top quality.
• Shop around. My experience is in London, but over here the prices in the diamond quarter and online are about the same. Beware of anyone who won't give you a straight price. Despite what anyone tells you, diamonds are close to commodities.
• Make sure you get a certificate, and don't buy anything with a non-GIA certificate.
• She will be wearing the ring all the time, and indefinitely into the future, which means there will be inevitable wear-and-tear. So platinum is probably the best metal.
• There is no reason to spend anything like the upper range of your budget. You can get an extremely nice (genuine) ring in the bottom half of that price range, and artificial will only be cheaper.