Let me give you some more immediately useful advice: a recommendation. The New York Diamond Center at 65 Broadway (ignore the big-sounding name, it's a small shop with an expert salesman) is where I bought both my engagement ring and our wedding bands. The salesman will provide useful and professional advice about the relationship between price, size, and quality. Importantly, he sells loose diamonds and orders the settings separately. You will get the GIA certificate from him for any diamond you buy. And then you also have your seller already picked out for wedding rings.

Do try to get an understanding of your girlfriend's taste before going to pick out a ring and a setting. It should not be a surprise or a secret that you're planning to propose, so talk about her taste in jewelry, whether she would prefer a more traditional round cut or one of the other more unique cuts, band color and design, etc. Also get her finger sized somewhere--even within average ranges, her finger could be anywhere from a 7 to a 9.

One important thing to remember on price is that it roughly scales with the square of the diamond's size in carats, so that a 1 carat diamond will probably be about 4 times the price of a 0.5 carat diamond. The guy I recommended will show you what diamonds of different quality look like so you can see what you're paying for.

Note that you can and should insure your diamond on your renter's or homeowner's policy. Your insurance broker will want a copy of your receipt and your GIA certificate.

One last tip, if you're planning to propose out of town and need to keep it hidden through airport security, sneakily transfer the box from your pocket to inside your shoe, and then back to your pocket on the other side of the scanner. And then make sure it's in the opposite pocket from your girlfriend on the flight so that the jewelry box digging into her hip doesn't give it away.