I learned several important things when getting a ring for my fiancee:
First off, gold is passe. At least yellow gold; get white gold or platinum. I remember distinctly watching an episode of Sex and the City with my gf (she's a fan, don't judge me) and one character gets a gold ring as an engagement ring and the women all get a little disgusted. I had no idea why and my gf had to explain.
Two, steal one of her existing rings to get her size from. Do not expect any other source will be forthcoming for this crucial information. For example, my gf told me to bring her friend for help when I go ring shopping. As much fun as I have hanging out with this girl (it definitely took a lot of the pressure off,) she was absolutely useless as far as any information about what to get. At the very least I was expecting she would know the size (since apparently she had been briefed ahead of time) but she didn't know.
Third, geographical location matters in terms of your girlfriend's feelings about these things. For example, I found out she intends to wear both the engagement ring and wedding band on one finger after we get married. I had never seen that before she told me, but since then I've been seeing that a lot. I don't know if the reason I never noticed it before was just because I wasn't paying attention or if it's a midwest thing (until recently I hadn't been around many midwest folks,) but at any rate it's something to keep in mind.
BONUS TIP: It may be too late to affect anything on this point, but making her think you are broker than you are will work to your advantage. In my case, her friend assumed I could only afford a tiny rock, and was relieved that I had enough saved to get a carat (incidentally, as another example of how useless she was, this girl had little understanding of diamond pricing until we got to the first store.) Lowered expectations are your friend here, assuming she really love you and would say yes to whatever.
To people who are saying "if she loves you she won't care how big the diamond is," what you have to realize is that she'll say yes regardless, but she might feel disappointment, and if YOU love her then you will want to make her happy.
man whatever you guys, when i get engaged my future husband had better be buying me one giant fucking diamond