I thought I was going to marry this one girl and did some research last year... maybe this will help you....
www.pricescope.com -- Gives you a tool that lets you price out a diamond based on market prices to see if your getting a good deal. Also has forums with various helpful vendors and people. I would deffinitely recommend reading through the threads to get an idea of the diamond market
http://www.goodoldgold.com/4csmain.htm -- This is a vendors site (not to far from where I live actually) that has a lot of great information about carat wieght, clarity, color and cut. As for online vendors (which are usually cheaper than say Zales) - they give you a lot of good information on the diamonds they are selling.
Good luck to you. I hope she says yes.