Ditto what DL said.
Try looking around for a smaller, mom and pop type ring shop to go looking first. Go in on a weekday (when it's less busy) or better yet - make an appointment. Be VERY up front about your budget. Then, let them educate you. Using DL's cheat sheet above, you'll know if the jeweler has his or her stuff together.
DON'T BUY A RING THAT DAY. They'll usually hold a couple of rings for you for a couple of days. Also, with rings, while every stone is different, value will be comparable. That is, price vs. cut, quality, size, clarity, etc. will usually be pretty consistent when you're talking about the same jeweler. They can usually print you out the gem certificate that you can use to compare prices at other ring shops, too.
Then, go check out the big box type jewelers and other mom and pop shops for comparisons, always referring back to that original certificate to remember those specs vs. price.
Bottom line, though, is to have fun shopping for a ring. What an exciting time. Depending on your girl, she may want to be involved in the ring shopping, too. So, no reason you've got to be the lone wolf on this one.
And last. Don't be afraid to ask lots and lots of questions. You probably asked more before you bought a couple of hundred dollar set of speakers. Why should it be any different when you buying a couple of thousand dollars worth of carved rock?
