Hey,
Others have made very good suggestions. If your girlfriend is so bold and fashionable, I'm afraid it might NOT be a good idea to buy a ring without her, based only on what you think she'll like. Some people really DON'T like yellow gold e-rings, for example. Maybe she really wants white gold or platinum?
Can you ask her, or her mom or sister, what color metal she would prefer for an e-ring?
I'm going to make a confession. Most of my friends and me are "realists" and I know that it's pretty common today for a guy NOT to totally surprise his gif and buy her a ring with her input (like go to the jewelry store WITH her, buy online with her, or ask her questions to know what she wants).
Ya, it does kill a 100% complete surprise. I would advise to give her some options, verbally, before spending thousands on something she may not be crazy about. if you're willing, you can go online with her to look at rings or just to get ideas. On many online sites, you can virtually design a ring and see how it looks like in white gold, yellow good, princess cut, round cut...
http://www.jamesallen.com/engagement-rings/
http://design.jared-diamonds.com/engagement-rings/?Nic=0
good guidelines, like other people said, are .75-1.0 carat, Princess/ascher/square/emerald cut (these are all square shapes).
Good luck! Let us know how it goes. I know for myself, personally, i would rather have input in what type of jewelry i get.