Hey,
I bought my fiancee a ring online and I HIGHLY HIGHLY recommend buying online to save money. She loves the ring. Even though I didn't know anything about diamonds or rings, it was super easy. Their customer service helped me pick out the diamond. I got mine online at James Allen
http://www.jamesallen.com/engagement-rings/ Their prices are great and they have free shipping and free returns. They also have very impressive crystal clear enlarged photos of the diamond and the ring. So you kinda know exactly what you're getting.
Of course you are nervous of forking over thousands of dollars online, but it's perfectly safe with a big well known company, Like James Allen or Blu nile. Don't ever buy from some obscure site no one has ever heard of.
Most people i know still buy at stores because they don't know about online jewelers or they're too afraid to buy a big ticket item online. With the good sites, it's very easy. And they have people you can call who will walk you through everything.
good luck and congrats dude!